Brush a 9x9-inch (23x23-cm) baking dish with the olive oil.
3. Step
In a medium bowl, whisk together the eggs, milk, and chives. Season with salt and pepper.
4. Step
Add the egg mixture to the baking dish and bake until just set in the middle, 15-20 minutes. Cut into 9 squares.
5. Step
For kids, layer one slice of cheese on top of one of the English muffin halves. Top with a frittata square. Spread the remaining English muffin half with mustard, and sandwich together.
6. Step
For adults, layer one slice of cheese on top of one of the English muffin halves. Top with a frittata square, roasted red pepper slices, and arugula. Spread the remaining English muffin half with mustard, and sandwich together.
