Melt butter in a saucepan over medium heat. Saute peas, onion, and garlic in hot butter until onion is translucent, 5 to 7 minutes.
Pour mushroom soup over the peas mixture; add enough milk to the mixture to thicken the soup to the point of there being no lumps. Flake tuna and add to the skillet; season with salt and pepper. Cook until the mixture is warm and bubbly, about 10 minutes.
