Preheat the oven to 175 C / 350 F. Grease a 12-cup muffin tin well with cooking spray or line with silicone liners.
Drain the canned salmon and check for any bones, then flake it into bite-size pieces with a fork.
Crack the eggs into a large mixing bowl with the milk. Whisk until fully combined and slightly frothy, about 1 minute.
Stir in the feta, chopped spinach, green onions, dill, salt, and pepper.
Fold in the flaked salmon gently, keeping some larger flakes for texture.
Divide the mixture evenly among the muffin cups, filling each about three-quarters full.
Bake for 18 to 20 minutes, until the muffins are puffed and the centers no longer jiggle when the tin is shaken. A toothpick inserted in the center should come out clean, egg dishes are safe once they reach 160 F / 71 C internally.
Cool the muffins in the tin for 5 minutes, then run a butter knife around the edges to loosen.
Serve warm, or let cool completely before storing.