Preheat oven to 350°F and grease an 8x8-inch casserole dish.
Melt the butter in a sauté pan. Sauté the onion until soft but not browned. Add the flour and blend with the onion. Add the milk, stirring constantly. Bring to the boil and boil for 2 or 3 minutes. Remove from heat.
Stir in the shredded cheese, peas, tuna and seasonings. Pour into the casserole. Cover with crushed corn flakes, dot with butter, and bake for 20-30 minutes.
Sprinkle parsley over the top of the hot casserole. Garnish with tomato wedges and lemon wedges. Serve hot.