Quinoa is quickly becoming a staple for Passover among Ashkenazim, despite objections of some on the grounds that it might be Kitniyoth (CYLOR, but as long as they are available and certified free of 5-grain contamination, I strongly feel they ought to be left out of that category, but I digress).
See the list of Star K certified kosher for Pesach Quinoa. (The OU apparently disagrees.)
In our year-round menu, we eat a bunch of quinoa dishes that are basically just rice dishes with quinoa instead of rice. Among them:
"Rice" (read: quinoa) and sauteed peppers.
Sautee chopped red peppers (we find green peppers tend to taste bitter when cooked or fried) in olive oil with fresh, chopped onion and crushed or chopped garlic.
Cook "rice" according to standard directions. (Add a bit of extra salt for quinoa.)
While "rice" is simmering, add the sauteed veggies.
For richer flavor, boil the "rice" with vegetable soup1 stock instead of water. Homemade stock is better, but if your friend is MSG-tolerant (I'm not), you can find bullion cubes that are KFP.
"Rice" and sauteed mushrooms.
Sautee sliced mushrooms in olive oil with fresh, sliced onion and garlic. Use Shitakes and Portabellos if you can find them. They tend to be chewier, which I kind of like. I also enjoy this mixture with the onions sliced rather than chopped (unlike above recipe). I'm not sure why.
See above cooking directions for the "rice".
While "rice" is simmering, add the sauteed mushroom mixture.
This is not on our standard, year-round menu, but it probably should be:
Your friend might not be interested in cholent because it's typically a meat dish, but you can veggify it easily into "cholent", and if your friend eats eggs, nothing beats a good "cholent" egg. I'd also add, from my perspective, that whenever we've had vegetarians for a meal, I always feel badly if I serve them something made just for them, like a piece of fish when everyone else is eating a piece of meat. I also try really hard to have something piping hot that they can enjoy like everyone else. Nothing wrong with a "vegetarian option", but I hate for it to be the "main dish" that they can't eat. My approach is to have basically nothing that is meat that can be singled out as "the main" dish. If there are meat items, I'll have things like meatballs or chicken wings or shnitzel, for example, making the veggie alternative seem much less isolated.
Choose your favorite cholent recipe and substitute quinoa for the usual barley. Double the amount, roughly, of the barley you would have used. (Eh, maybe not quite double.)
Add carrots and/or sweet potato (if you don't normally have them in your cholent), as a substitute for the verboten kidney beans (Kitniyoth). You won't miss the beans anyway, since quinoa packs a lot of protein and fiber of its own.
Don't forget the potatoes. I'd use more than usual, but if you've got other veggies in there, you might not need to.
For flavor, we typically use fresh garlic, cut into chunks (seems to impart flavor better than whole), onion powder or fresh onion, depending on your taste, about a tablespoon or so of salt, a blended mixture of barbeque sauce (smokey and/or honey if you can find them KFP) and "mustard" (I know, KFP mustard is not amazing, but it still has that zing we like, so it works for a cholent or a "cholent"), a couple of tablespoons of each, and honey. Honey is our secret ingredient in all our cholents, so don't tell anyone! ;-)
Finally, try to get your hands on a parve, KFP Kishke. It exists, and it adds a nice flavor to any cholent.
As mentioned above, if your friend eats eggs, just put 4 or 5 (depending on the size of your pot) whole raw eggs into the cholent; nest2 them on something so they don't roll around and crack.
Add water just to cover the tallest ingredient. However, if that tallest ingredient is an egg or the Kishke, it's ok if it pokes out above the water line a bit. Watery cholent is more of a risk with quinoa than with barley. If you think you might have too much water, you can also get KFP couscous (seriously!) which can help absorb the excess water.
And there you have it. Just don't add any meat!
You'll likely want to cook it on the stovetop or in the oven instead of a slow-cooker, unless you've got your slow-cooker set on a timer and/or you plan on using it again for another meal. Keeping a slow-cooker going for 3 days straight is probably not a great idea in general. If you use a slow-cooker, cook it on high for an hour or so, unless you add boiling water at the outset, and then let it simmer for 12+ hours.
For stovetop or oven cooking, take this recipe over to Seasoned Advice!
1 Speaking of vegetable soup, there's another dish right there!
2 No pun intended.