Making the Spinach Pesto

1. In a blender or food processor, add baby spinach, olive oil, walnuts, parmesan cheese, salt, and pepper.
2. Blend until smooth, scraping down the sides if necessary. The consistency should be spreadable but not too runny.
3. Taste and adjust seasoning if needed. If it is too thick, add a small drizzle of olive oil and blend again.
4. Set the pesto aside. If making it in advance, store it in an airtight container in the refrigerator for up to three days.

Assembling the Sandwiches

1. Lay out four slices of bread. Butter one side of each slice. The buttered side will be the outer layer when grilling.
2. On the unbuttered sides, spread about two tablespoons of spinach pesto per slice.
3. On two slices, layer two slices of smoked gouda.
4. Add ¼ cup baby spinach leaves on top of the cheese.
5. If adding chicken or beef, place a thin layer of cooked meat over the spinach.
6. Place another slice of gouda over the spinach and meat, then cover with the remaining bread slices (pesto side down, buttered side up).

Grilling the Sandwiches

1. Heat a skillet or nonstick pan over medium heat.
2. Place the sandwiches in the pan and cook for 3 to 5 minutes. Watch closely to prevent burning. The goal is to achieve a golden-brown crust while allowing the cheese to start melting.
3. Flip the sandwiches carefully and cook the other side for 1 to 2 minutes. Since the pan will already be heated, the second side cooks faster.
4. Once both sides are crispy and golden brown, and the cheese is fully melted, remove the sandwiches from the skillet.
5. Let them rest for a minute before slicing in half and serving.