Prepare all the ingredients.
Sauté the Fresh Spinach (0.25 pound).
Cut 2 thick slices of the Sourdough Bread (2 slice).
Spread 1 slice of bread with Unsalted Butter (1/2 tablespoon).
Place butter side down on the cutting board. Top unbuttered side of the bread with grated Gruyère Cheese (1.5 ounce).
Add grated Cheddar Cheese (1.5 ounce).
Add Pancetta (3 slice).
Add sliced Brie Cheese (1.5 ounce) and sprinkle with Ground Black Pepper (to taste).
Place second slice of bread on top and spread with the other Unsalted Butter (1/2 tablespoon).
Heat small nonstick skillet over medium heat. Add Olive Oil (1 tablespoon). Once oil is hot but not smoking, place the sandwich in skillet.
When underside is golden brown, about 4 minutes, turn sandwich and cook for another 4 minutes until both sides are golden brown.
Cut in half and serve immediately, while cheese is still melty.