When in doubt, we look to precedents. In this case, we find one in Ezekiel 3:1-3:
וַיֹּ֣אמֶר אֵלַ֔י בֶּן־אָדָ֕ם אֵ֥ת אֲשֶׁר־תִּמְצָ֖א אֱכ֑וֹל אֱכוֹל֙ אֶת־הַמְּגִלָּ֣ה הַזֹּ֔את וְלֵ֥ךְ דַּבֵּ֖ר אֶל־בֵּ֥ית יִשְׂרָאֵֽל׃ וָאֶפְתַּ֖ח אֶת־פִּ֑י וַיַּ֣אֲכִלֵ֔נִי אֵ֖ת הַמְּגִלָּ֥ה הַזֹּֽאת׃ וַיֹּ֣אמֶר אֵלַ֗י בֶּן־אָדָם֙ בִּטְנְךָ֤ תַֽאֲכֵל֙ וּמֵעֶ֣יךָ תְמַלֵּ֔א אֵ֚ת הַמְּגִלָּ֣ה הַזֹּ֔את אֲשֶׁ֥ר אֲנִ֖י נֹתֵ֣ן אֵלֶ֑יךָ וָאֹ֣כְלָ֔ה וַתְּהִ֥י בְּפִ֖י כִּדְבַ֥שׁ לְמָתֽוֹק׃
And He said unto me: 'Son of man, eat that which thou findest; eat this scroll, and go, speak unto the house of Israel.' So I opened my mouth, and He caused me to eat that scroll. And He said unto me: 'Son of man, cause thy belly to eat, and fill thy bowels with this scroll that I give thee.' Then did I eat it; and it was in my mouth as honey for sweetness.
What Ezekiel ate was not a Torah scroll, but nevertheless, it was a God-given scroll, so we can make an analogy from here. We see that this scroll was "as honey" (כדבש).
There is a makhloket as to what extent the scroll is "as honey". There are some who say that it was like honey mamash, and thus the bracha would be שהכל. There are some who say no, but rather: where else do we find that some food item has a flavour "like honey"? By the manna that the Jews ate in the desert, as it's written (Exodus 16:31):
וַיִּקְרְא֧וּ בֵֽית־יִשְׂרָאֵ֛ל אֶת־שְׁמ֖וֹ מָ֑ן וְה֗וּא כְּזֶ֤רַע גַּד֙ לָבָ֔ן וְטַעְמ֖וֹ כְּצַפִּיחִ֥ת בִּדְבָֽשׁ׃
And the house of Israel called the name thereof Manna; and it was like coriander seed, white; and the taste of it was like wafers made with honey.
So by these analogies, it follows that the bracha we should say on eating a Torah scroll is the same as that which we said on manna, viz. הממתיר/המוציא לחם מן השמים.