Prep the tuna: In a bowl, mix together the tuna, mayonnaise, diced cucumber, and red onion. Season with salt and pepper to taste. Set aside.
Toast the base: Preheat your oven to 400°F (200°C). Place the sourdough slices on a baking tray, top each with a slice of cheddar, and bake until the bread is crisp and the cheese is melted and bubbly.
Assemble: Remove from the oven and top each slice with a handful of mixed greens or spinach. Spoon the tuna mixture generously over top.
Serve: Serve right away while the bread is warm and the cheese is still melty.