In a cast iron skillet, over medium-high heat, cook the bacon until brown, then remove the bacon and pour off all but a tablespoon of bacon fat. Place the pan back over the heat, add onion and cook until soft (about 2 minutes), then add peas and cook for another 4 minutes. Remove from heat, season to taste with salt and pepper, and set aside.

Brush each side of the sliced bread with mayo, then place in the cast iron skillet over high heat, toasting until both sides of the bread are golden brown. Then remove from heat. Add butter to the pan and place back on the heat, frying the eggs until whites are set and the yolks are runny.

Divide the pea and bacon mixture between the two slices of bread, then top each piece with an egg and serve.