Place the bacon slices in a cold skillet and turn the heat to medium. Cook the bacon, flipping every minute or so, until the fat is rendered and the bacon is crispy. Remove the cooked bacon from the skillet and set aside on a paper towel to drain excess grease. I find that starting with a cold pan helps the bacon cook more evenly and get crispier.

While the bacon is cooking, slice the heirloom tomato into approximately 1/4 inch thick slices. Set aside until ready to assemble the sandwiches.

In a small bowl, mix together the mayonnaise and generous pesto until well combined. This creates a flavorful aioli that will be spread on the sourdough bread. I like to make extra aioli for dipping later—it’s just that good!

Lay out the slices of sourdough bread and spread a hearty layer of pesto aioli (from Step 3) on each piece. On half of the slices, layer the sliced mozzarella cheese, followed by the tomato slices (from Step 2), cooked bacon (from Step 1), and finish with a handful of arugula or baby spinach. Top each with the remaining slices of bread, aioli side down.

Preheat a panini press to medium-high. Place the assembled sandwiches in the press and cook until the cheese is melted and the bread has lovely golden brown grill marks, about 3–5 minutes. If you don’t have a panini press, you can grill them in a skillet, pressing down with a heavy pan.

Serve the grilled sandwiches hot, with additional pesto aioli (from Step 3) on the side for dipping. Enjoy—dipping makes these even more delicious!