Season both sides of the walleye with salt and freshly ground black pepper. Coat both sides of the walleye fillet with flour, tapping off the excess. Heat the butter and oil in a nonstick skillet over medium-high heat and cook the walleye on one side for 2 to 3 minutes, or until golden. Turn the fillets over and cook until fish is opaque and cooked through. Remove to a plate and keep warm. Add the mushrooms and diced tomatoes to the fish pan and cook until mushrooms are golden and most of the liquid has evaporated. From sauce ingredients add butter, garlic and flour. Stir and cook for about 1 minute. Add the rest of the ingredients and stir for about 3 minutes, until smooth and hot. Serve the fish over linguini and top with the pan sauce. Garnish with a lemon wedges.