Step 1: Prepare the Sausage - Slice the Andouille sausages into thin rounds for even cooking.
Step 2: Cook the Sausage - In a skillet over medium heat, cook the sliced Andouille sausage for 5-7 minutes until browned. Remove from heat.
Step 3: Assemble the Sandwich - Spread Dijon mustard on one side of 4 slices of bread. Layer cheddar, sausage, and Monterey Jack on the unbuttered side, then top with another slice of bread.
Step 4: Butter the Bread - Spread softened butter on the outer sides of each sandwich.
Step 5: Grill the Sandwich - Heat the skillet and grill the sandwiches for 3-4 minutes on each side until golden brown and cheese is melted.
Step 6: Serve and Enjoy - Slice the sandwiches in half and serve immediately, ideally with a side of soup.