Preheat the oven to 400 degrees F.

Heat the olive oil over medium-high in a large skillet and add the mushrooms and bell pepper. Saute, stirring occasionally until mushrooms have softened and begin to brown, about 3 to 5 minutes. Add the garlic, yellow squash, zucchini squash, black beans, and green onion and continue cooking, stirring occasionally, until garlic is fragrant, about 3 minutes.

Spread ¼ cup of enchilada sauce on the bottom of a large casserole dish. Add about ½ cup of enchilada sauce to a wide-mouth bowl. Dip a tortilla into the enchilada sauce to coat. Flip and coat the other side in sauce. Add about ¼ to ? cup of the vegetable mixture to the center of the tortilla and roll to create an enchilada. Place in the prepared casserole dish and repeat with the remaining tortillas and filling.

Pour the rest of the enchilada sauce over the prepared enchiladas and evenly layer the shredded cheese on top. Bake on the center rack of the preheated oven (uncovered) for 20 minutes, or until cheese has melted and enchilada sauce is bubbling. Remove from the oven and allow enchiladas to cool at least 10 minutes before serving. Serve with sliced black olives and chopped cilantro.