Preheat oven to 350°. In large skillet, sauté the onions and cashews with 2 tbsp olive oil over medium/high heat. Season with salt and pepper and sauté until onions are soft and lightly browned. Add garlic and let cook a few more minutes. In a large bowl combine onion mixture, brown rice, lentils, breadcrumbs, vegetable broth, basil, and thyme. Mix together and season to taste with salt and pepper.
Brush both sides of mushrooms caps lightly with olive oil and place on an oiled sheet pan. Stuff mushrooms with about 1/2 cup lentil-cashew stuffing and then press one tomato slice on top of the stuffing.
Bake for approximately 30 minutes or until the stuffing is browned and the mushroom begins releasing juices. Garnish with extra fresh thyme leaves and let the feast begin!
Do-ahead Tip: The stuffing can be made up to 3 days in advance and stored covered in the refrigerator. The mushrooms can be stuffed and assembled on a baking tray 1 day before you bake them.