Have the water heating in a teakettle or pan so it will be boiling when you're ready to add it to the farro. Use a heavy pan with a tight-fitting lid. Add the farro to dry pan and toast over medium-high heat until it starts to look and smell toasted, about 3 minutes. Turn off heat, and then carefully pour in the boiling water (it will boil up and sputter, so be careful.) Add the salt, then turn heat back on to a low simmer, cover pan, and let cook until the farro is tender, but chewy, about 50 minutes. (I would start checking after about 40 minutes. You may need to add a tiny bit more water.)
Use a large heavy frying pan for the second part of the recipe. Heat the oil, then add the shallot and garlic and sauté until onions are starting to soften, about 3 minutes.
Add sliced mushrooms and stems, sprinkle with a pinch of salt, and sauté over medium high heat until mushrooms have released their liquid and are well browned, about 5 minutes (or longer, depending on your pan).
Stir in the balsamic vinegar/water mixture and cook until liquid is mostly evaporated, about 2 minutes. Add cooked farro and heat 2-3 minutes, stirring gently. If your pan is not non-stick, you may need to add a bit of water when you heat the farro. When farro is heated through and moistened, add the thyme, stir, season with salt if desired and serve hot.
Adapted from Kalyn's Kitchen