For the gravy, in a large sauté pan over high heat, sauté mushrooms in olive oil until mushrooms release all their liquid and begin to brown, 15-20 minutes. Remove mushrooms to a plate; set aside. In the same pan over medium-low heat, sauté onion, yeast, sage, garlic, paprika, pepper flakes, fennel seed and coriander for 5 minutes. Stir or whisk in ground chia seeds and 3 cups soymilk. Let gravy simmer over medium heat until thickened to desired consistency, about 8 minutes. Stir in soy sauce and mushrooms. Season with salt and pepper.