Grilled Sausage Sandwich with Organic Cheese

1. Heat oil in large, heavy skillet over medium-high heat. Add onions and sauté until softened, about 5 minutes.
2. Stir in vinegar and sugar. Reduce heat to medium-low and cook until onions are soft and dark brown, about 20 minutes.
3. Season to taste with salt and pepper.

(You can prepare this up to 2 hours ahead. If you do, hold at room temperature, then rewarm over low heat before using.)

1. Preheat broiler. Brush large metal baking pan with oil.
2. Place sliced sausages in pan. Broil until sausages are brown and crisp, about 5 minutes per side. Sausages may also be grilled over medium-high heat.
3. To assemble sandwiches, place 4 slices of bread on work surface. Top each slice with one ounce of cheese.
4. Place 3 sausage halves on each slice, and spread 1 teaspoon of mustard on top of sausages.
5. Top sausages with caramelized onions and remaining cheese slices, dividing equally. 
6. Finish with remaining 4 bread slices. Spread half of butter over upper bread slices.
7. Heat large nonstick skillet over medium-low heat, or electric griddle on medium.
8. Very carefully ... invert sandwiches, buttered side down, onto skillet or griddle.
9. Cook until bottoms are brown, about 3 minutes. Spread remaining butter over upper bread slices.
10. Turn sandwiches, and cook until bottoms are brown, about 3 minutes longer.