Gather all ingredients on a clean surface.
Drain the can of tuna and place it in a mixing bowl.
Add diced red onion to the bowl with the tuna.
Halve the cherry tomatoes and add them to the mixture.
Pour in the olive oil and lemon juice.
Season with salt and pepper to taste.
Gently mix all ingredients until well combined.
Prepare a plate with fresh spinach as the base.
Spoon the tuna mixture onto the spinach.
Garnish with additional toppings if desired.