Prepare the Bacon: Place the streaky bacon in a cold, non-stick frying pan. Turn the heat to medium and cook for 8-10 minutes, turning occasionally, until it's perfectly crisp. Transfer the bacon to a plate lined with a paper towel to drain the excess fat. Carefully wipe the pan clean.

Grate the Cheeses: While the bacon is cooking, grate your mature cheddar and mozzarella into a medium bowl. Toss them together with the freshly ground black pepper to combine. This ensures you get a good mix of both cheeses in every bite.

Assemble the Sandwiches: Lay the four slices of sourdough on a clean work surface. Spread the softened butter evenly on one side of each slice. This will be the outside of your sandwiches.

Layer the Fillings: Flip two of the bread slices over (buttered-side down). Spread 2 tablespoons of basil pesto on the un-buttered side of each.

Add the Good Stuff: Sprinkle half of the cheese mixture over the pesto on each slice. Top with three slices of crispy bacon each, then scatter the chopped sun-dried tomatoes over the bacon. Finish with the remaining cheese mixture, dividing it evenly between the two sandwiches.

Close and Cook: Place the remaining two slices of bread on top, buttered-side up. Gently press down.

Pan-Fry to Perfection: Heat the cleaned frying pan over a medium-low heat. Carefully place the assembled sandwiches in the pan. Cook for 5-6 minutes on the first side, until the bread is a deep golden brown. I find that covering the pan with a lid for the first 2 minutes helps trap the steam and gives the cheese a head start on melting.

Flip and Finish: Using a wide spatula, carefully flip the sandwiches. Cook for another 4-5 minutes on the second side, until it's equally golden and the cheese is completely melted and oozing from the sides.

Rest and Serve: Remove the sandwiches from the pan and let them rest on a cutting board for one minute. This allows the molten cheese to set just slightly, preventing it from spilling out everywhere when you cut it. Slice in half diagonally and serve immediately.