Cut the eggplant into 1-inch cubes; do not peel. Use red, green or gold sweet bell peppers, or a mixture; slice thin.
Place eggplant cubes in a shallow baking pan, toss with 1/2 cup olive oil, sprinkle with salt to taste, and bake in a 400 degree F oven until cubes are soft but still hold their shape, about 30 minutes. Set aside to cool.
Heat 1/4 cup olive oil in a large heavy frying pan. Add onions and sweet peppers and cook over very low heat until vegetables are quite tender and slightly caramelized, about 35-45 minutes. Stir in garlic and salt and pepper to taste; cook 5 minutes longer.
Combine the remaining 1/4 cup olive oil, lemon juice, herbes de provence, and salt and pepper to taste. Whisk and reserve.
Cook penne in 4 quarts boiling water until very al dente. Drain and rinse well in cold water, then drain again. With a sharp knife, diagonally slice each piece of pasta in half crosswise. Toss in a large bowl with onion mixture and reserved dressing.
Toss eggplant cubes and chopped basil with pasta. Garnish with basil leaves.
Serves 6 to 8 as first course; 10 to 12 as a salad.