1. On waxed paper, combine flour, salt, and pepper. Cut eggplant into 1/4 inch-thick slices; dip eggplant slices into flour mixture to coat them.
2. Preheat oven to 350°F. In 12-inch skillet over medium-high heat, in 1/4 cup hot salad oil, cook eggplant, a few slices at a time, until tender and browned on both sides, removing slices to plate as they brown and adding more oil as needed.
3. Into four 16-ounce oven-safe shallow casseroles or ramekins, spoon half of spaghetti sauce; arrange half of eggplant slices on sauce. Top with ricotta cheese, remaining eggplant slices, then mozzarella cheese and remaining spaghetti sauce. Bake casseroles 20 minutes or until cheese melts and mixture is hot and bubbly. If you like, serve with grated Parmesan cheese.