1. Blot beans dry with paper toweling. Combine beans, jalapeno Jack cheese and cumin in a bowl.
2. Spread half the bean-cheese mixture over 4 slices of bread.
3. Brush avocado slices with lemon juice and place on bean-topped bread slice, dividing equally. Top each bread slice with 3 arugula leaves. Spread remaining bean-cheese mixture over arugula and top each sandwich with remaining bread slices.
4. Melt 2 teaspoons of the butter in a large nonstick skillet over medium-low heat. Place 2 sandwiches in the skillet and cook for about 2 to 3 minutes or until browned. Using a wide metal spatula, carefully turn sandwiches over and cook, pressing down slightly on top of each sandwich, for 2 to 3 minutes or until browned. Remove the sandwiches from the skillet and keep warm. Repeat with the remaining butter and sandwiches.