Toast bread slices in a toaster. Top each slice of toast with 2 slices of cheddar. Top cheddar with baby spinach, tuna, tomato and red onion. Top each slice of toast with one more slice of cheddar.
Place toast on an oven safe plate and put under the broiler, keeping a close eye on it so the toast doesn't burn. Broil until cheese is melted and begins to bubble and lightly brown. Remove from oven. Top with freshly ground pepper. Serve.
If you can't find tuna canned with pesto, you can toss a can of tuna in water (or olive oil) with homemade or store-bought pesto.