Add all ingredient for pesto (abovin a food processor or blender, blend until smooth, scraping sides down if needed. Set aside.

Set out 4 slices of bread and butter the outside facing sides. On the inside of the 4 slices, slather spinach pesto (about 2 TBS per side), then top with 2 slices of gouda on half. Add ¼ cup baby spinach leaves, then place 1 piece of gouda on top, and place the remaining slice of bread on top (spinach pesto side down, butter side up). Repeat for second sandwich.

In a skillet over medium heat, place sandwiches and cook for 3-5 minutes, or until bottom is browned. Watch closely as you don't want to burn them, but you want the cheese to start melting. Do not turn the heat to high! Flip sandwiches and cook for 1-2 minutes on opposite side (will cook faster), remove, cut in half and serve immediately!