Step 1

Preheat the oven to 375 degrees F.

Step 2

In a large ovenproof skillet, heat 2 tablespoons olive oil and 2 tablespoons butter over medium heat. Add the onions, garlic and thyme. Cook, stirring, until the onions become a deep golden brown, 14 to 16 minutes. Add the balsamic vinegar, Worcestershire sauce and a good pinch of pepper. Stir to combine and remove to a plate.

Step 3

Melt the remaining 2 tablespoons butter in the skillet. Add the potatoes and cook, stirring, for 2 to 3 minutes to start the cooking process. Press into a thin layer across the bottom of the skillet. Turn the heat to low. Sprinkle over 1 1/2 cups Gruyère, then top with the caramelized onions.

Step 4

Sprinkle the eggs with a good pinch of salt and pepper. Pour the eggs into the skillet, filling all the nooks. Sprinkle over the chives. Transfer to the oven and bake until set and lightly golden on top, 20 to 22 minutes.

Step 5

Meanwhile, lay the slices of bread on a sheet pan. Drizzle with the remaining 2 tablespoons olive oil. Divide the remaining 1/2 cup cheese among the slices. Place in the oven to toast the bread and melt the cheese, 10 to 12 minutes.

Step 6

Remove the frittata and let sit for 5 minutes before slicing into wedges. Serve the wedges with a slice of cheesy toast and a few tomato slices on the side.

Step 7

Make ahead instructions: To make ahead of time, wait to bake the cheesy toast until ready to serve. Let the cooked frittata cool completely, 25 to 30 minutes, before covering with foil and refrigerating. To serve, bake the cheesy toast alongside the foil-covered frittata in a 375 degree F oven until the frittata is heated through, the bread is toasted and the cheese is melted, 10 to 12 minutes.