Eggplant Spaghetti Sauce

Melt butter in a large skillet over medium heat. Add onions and garlic; cook and stir until tender, about 5 minutes. Add eggplant, red bell pepper, green bell pepper, celery, and mushrooms; cook and stir until eggplant is tender but not mushy, about 10 minutes. Pour in tomato sauce. Simmer until bubbly and thick, about 15 minutes.