In a large skillet, heat 3 tablespoons of olive oil over medium heat. Add the chopped baby bella mushrooms and red bell pepper. Sauté for about 5 minutes until the mushrooms are tender and browned.
Stir in the minced garlic, yellow squash, and zucchini squash. Cook for another 5-7 minutes until the squash is tender.
Add the drained black beans and chopped green onion to the skillet. Mix well and cook for an additional 2-3 minutes to heat through. Season with salt and pepper to taste.
Preheat your oven to 375°F (190°C). While the oven is heating, warm the flour tortillas in a dry skillet or microwave for easier rolling.
Spread a thin layer of red enchilada sauce in the bottom of a 9x13-inch baking dish. Take one tortilla, spoon about 1/4 cup of the vegetable filling onto the center, sprinkle some grated cheese, and roll it up tightly. Place seam-side down in the baking dish. Repeat with remaining tortillas and filling.
Once all the enchiladas are in the baking dish, pour the remaining red enchilada sauce over the top, making sure to cover each enchilada. Sprinkle the remaining grated cheese over the sauce. Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove the enchiladas from the oven and let them cool slightly. Top with sliced black olives and fresh cilantro before serving. Enjoy your delicious Mushroom, Zucchini, and Black Bean Vegetarian Enchiladas warm!