1 cup chopped yellow onion (1 onion)
1 cup chopped fennel (1 bulb)
3 tablespoons good olive oil
1 teaspoon minced garlic
28 ounces canned plum tomatoes, drained
1 teaspoon kosher salt
¾ teaspoon freshly ground black pepper
2 tablespoons pareve chicken stock or vegetable stock
2 tablespoons good dry white wine
½ cup chopped fresh basil leaves
2 tablespoons capers, drained
1 tablespoon unsalted margarine
4 (1-inch thick) flounder fillets (about 2 ½ pounds)
Fresh basil leaves
For the sauce, cook the onions and fennel in the oil in large sauté pan on medium-low heat for 10 minutes, until the vegetables are soft. Add the garlic and cook for 30 seconds. Add the drained tomatoes, smashing them in the pan with a fork, plus the salt and pepper. Simmer on low heat for 15 minutes. Add the chicken stock and white wine and simmer for 10 more minutes to reduce the liquid. Add the basil, capers, and margarine and cook for 1 minute more.
Prepare a grill with hot coals. Brush the fillets with olive oil, and sprinkle with salt and pepper. Grill on high heat for 5 minutes on each side until the center is no longer raw. Do not overcook. Place the sauce on the bottom of a plate, arrange the flounder on top, and garnish with basil leaves. Serve hot or at room temperature.