Rinse fish; pat dry. Sprinkle fish fillets with the salt and pepper. In a 12-inch skillet, combine the 2/3 cup Fish Stock, the white wine, sprigs of mint, oil, and butter. Bring to boiling; reduce heat. Add fish fillets. Cover and simmer gently for 8 to 10 minutes or until fish flakes easily with a fork, adding the yellow squash and zucchini the last 2 to 3 minutes of cooking.
With a slotted spatula, transfer fillets, squash, and zucchini to oval serving dishes or dinner plates; keep warm. Discard cooked mint.
Bring the skillet juices to boiling over high heat; continue cooking for about 6 minutes or until the sauce is thickened and reduced to 1/2 cup. Add the julienne-cut mint and lemon juice. Pour over the fillets. Makes 4 servings.
Prepare the stock as directed. Refrigerate for 1 day or freeze for longer storage.