Preheat oven to 350°F. In a large ovenproof skillet (cast iron is perfect!) heat a drizzle of oil and cook the onion over medium-high heat for a few minutes, until starting to soften. Squeeze the sausage out of its casing into the skillet and cook, breaking it up with a spoon, until no longer pink. Transfer to a bowl and add another drizzle of oil (if you need it) then add the chard to the pan - if you've washed it, the moisture clinging to the leaves should provide enough moisture - and cook for a few minutes, until it wilts. Add the sausage and onion back to the pan.
Whisk eggs, milk, salt and pepper. Stir in the cheese and pour over the meat and veg in the pan. Turn the heat to medium-low and cook for a few minutes, until it's starting to set around the edges and on the bottom. Transfer to the oven and cook for about 10 minutes, until cooked through and golden on top. (Alternatively you could stir the lot together, pour it into a buttered baking dish and bake for 40-45 minutes, until set.)
Serve warm, in wedges, or cold in squares. Leftovers make great sandwich filling.