In a large, heavy-bottomed saucepan, heat the oil over medium heat. Add the onions, bell peppers and celery. Season with salt and cayenne and black pepper. Cook, stirring constantly, for about 3 minutes, or until the vegetables are slightly wilted. Add the eggplant. Season with salt and cayenne. Cook for 4 to 5 minutes, or until slightly tender. Add the zucchini, yellow squash, tomatoes, garlic, basil and thyme. Season with salt and cayenne. Reduce heat to medium and cook, stirring occasionally, for 8 to 10 minutes, or until the mixture is a little soupy. The vegetables should have a little crunch to them. Remove from the heat. Season both sides of the fillets with salt and pepper. Spread both sides of the fillets evenly with the mustard. Dredge the fillets in the herbs, coating each side completely. In a large saute pan, over medium heat, add the oil. When the oil is hot, add the fillets and cook for about 3 to 4 minutes on each side, or until the fish is flaky.
To serve, spoon some of the ratatouille in the center of each plate. Place the fillets on top of the ratatouille. Garnish with parsley.