Start by softening your unsalted butter or mix it with mayonnaise to create a flavorful spread. Grate the low-moisture mozzarella, provolone, and Parmesan cheeses and combine them in a bowl. Chop your desired toppings like bell peppers and red onion, and measure out the marinara sauce to have everything ready for assembling your Ultimate Pizza Grilled Cheese.

Take four slices of your sturdy bread and spread the buttery mixture generously on one side of each slice. On the unbuttered side of those four slices, spread a layer of marinara sauce, then layer with a generous portion of the cheese mixture followed by your chosen toppings. Top each sandwich with another slice of bread, but with the buttered side facing outward.

Preheat a medium-low skillet or non-stick pan over a gentle flame. Once hot, place the assembled sandwiches butter-side down in the pan. Grill each sandwich for 4-6 minutes, or until the bread reaches a golden-brown hue and the cheese begins to melt. Cover the pan with a lid for a minute to help the cheese melt evenly.

Carefully flip each sandwich using a spatula and continue grilling for an additional 4-6 minutes until the other side is golden and the cheese is visibly bubbled and gooey. Keep an eye on the sandwiches to avoid burning, and feel free to adjust the heat if necessary to achieve that perfect grilled look.

Once your Ultimate Pizza Grilled Cheese is done, transfer them to a cutting board and let them rest for about 2-3 minutes. This helps the cheese set slightly, making them easier to slice. Diagonally cut the sandwiches and serve them warm, alongside a small bowl of extra marinara sauce for dipping, and sprinkle with red pepper flakes for an added kick if desired.