Slice zucchini and squash in half lengthwise and then into 1/4-in slices. Add oil to a large skillet, turn to medium heat, and sauté onion, mushrooms, zucchini, squash, and bell pepper until desired tenderness. I prefer to cook the veggies just until tender when pierced with a fork, about 3 to 5 minutes. Add black beans, spices, and salsa. Stir and cook until warm, about 2 minutes.
Sprinkle with cheese, cover, and cook just until melted, about 2 minutes. Sprinkle with diced tomatoes and green onions. Remove from heat and serve in individual dishes or add to taco shells or tortillas. For healthy tortillas, try our Homemade Whole Grain Tortillas.