Preheat the oven to 350 F / 175 C. Grease a 12-cup muffin tin (2.5-inch cups) with cooking spray or butter, or line with silicone liners.
Dice the ham steak into small 1/2 cm cubes and dice the bell pepper. Thinly slice the green onions and set aside.
In a large mixing bowl, whisk the eggs, milk, salt, and pepper for about a minute, until pale and slightly frothy.
Divide the diced ham and bell pepper evenly among the 12 muffin cups. Add about two-thirds of the cheddar, saving the rest for the top.
Pour the egg mixture over the fillings, filling each cup about three-quarters full.
Scatter the remaining cheddar over the tops of the muffins.
Bake for 18 to 20 minutes, until the centers are set and no longer jiggle when you nudge the pan and the edges turn golden brown.
Remove from the oven and scatter the sliced green onions over the hot muffins.
Cool in the tin for 5 minutes, then run a knife around the edges and transfer the muffins to a wire rack.