Preheat oven to 350°F and grease or butter a medium casserole or 8x8" baking pan.
In a heavy-bottomed skillet over medium heat, heat 1 tablespoon olive oil or vegan butter. Add in sliced mushrooms and sprinkle with salt to draw out moisture. Sauté for 5-8 minutes, or until most of the excess liquid has cooked off.
Add in diced onions and sauté for an additional 3-5 minutes, until the mixture begins to brown. Add in minced garlic and sauté for another minute.
Add in 3 tablespoons vegan butter and stir to melt and coat vegetables.
Sprinkle flour over mixture and cook for 1-2 minutes, stirring constantly. The flour mixture may stick to the bottom of your pan slightly; this is okay! It'll produce a lot of flavor.
Add white wine to deglaze, scraping and stirring any browned bits from the bottom of the pan.
Stir in broth and cashew cream/plant milk and bring to a boil. Reduce heat and simmer until thickened. Add in fresh or dried thyme. Remove from heat. Taste and season with salt and pepper to preference.