Arrange green beans on one large sheet tran and mushrooms on a second sheet tray. Toss each with olive oil, salt, pepper, and scatter fresh thyme sprigs on top (2 tbsp olive oil for each sheet tray). Roast in oven for 15-20 minutes or until green beans are charred and mushrooms are browned. Remove from oven and set aside.
In the meantime, soak cashews in a bowl of water, set aside.
Heat a large skillet over medium-high heat. Add 1 tbsp olive oil and sliced onion. Cook until onion is soft and "melty" about 15 minutes. Do not allow the onions to brown, add stock to the pan as you are cooking to help the process along.
When onions are soft and tender add them to a high speed blender with the drained cashews and 1/2 cup of water. Blend until super creamy and thick, resembling heavy cream.
Arrange green beans and mushrooms on a platter and drizzle the creamy onion sauce on top. Serve immediately or store in fridge for 3-4 days (store the green beans and mushrooms separate from the onion sauce)
VEGAN CREAMY ONION SAUCE:You will likely have some leftover creamy onion sauce, this is a GOOD problem to have. Here are some other ways to use the sauce:
Use it as vegan gravy - Rich onions make the perfect base for a vegan gravy, pour this on mashed potatoes or stuffing!
Use as a base for "creamed greens" - Simply saute a combination of spinach and kale (or any dark leafy green you love) with a lot of fresh garlic. Once the greens have wilted add the creamy vegan sauce to make the BEST creamed spinach you've ever had
Use it as a "cream" for soup - Creamy tomato soup, carrot soup, cauliflower soup, or broccoli soup. Cook your veggies, add sauce, blend and savor!