Description of the issue
The steel block of the IE is replaced by the steel block of Mekanism,
but I cannot craft the Steel Slab by using Mekanism's steel block.

Versions & Modlist
Minecraft 1.15.2
Forge 31.2.20
ImmersiveEngineering-0.15-103
Mekanism-1.15.2-9.10.7.420
MekanismGenerators-1.15.2-9.10.7.420
MekanismTools-1.15.2-9.10.7.420

This is intentional.
IE's metal slabs are crafted only with IE's metal blocks and do not use tags. The reason for that is that I expect other mods to add slabs, and I wouldn't want to override their recipes with mine.

Either get your hands on IE steel blocks (crafted from IE steel ingots) or tell Mekanism to add slabs.

This is intentional.
IE's metal slabs are crafted only with IE's metal blocks and do not use tags. The reason for that is that I expect other mods to add slabs, and I wouldn't want to override their recipes with mine.

Either get your hands on IE steel blocks (crafted from IE steel ingots) or tell Mekanism to add slabs.

I can't craft IE steel blocks with IE steel ingots, the output is still Mekanism's steel block.
On the other hand, since there is no way to get Block of Steel except the \give command, even the Arc Furnace is Impossible to build if Mekanism is enabled.

I can't craft IE steel blocks with IE steel ingots, the output is still Mekanism's steel block.

Well maybe mekanism shouldn't craft their steel block with tags then?! They should make their metal blocks use their ingots, instead of just overriding all other mods.
That's still on their side to fix, not ours.