Caramelized Onion & Asparagus Tart | America's Heartland

Ingredients

• 2 tsp butter (or olive oil)
• 3 large onions, cut lengthwise and sliced root to end
• 1 tbsp sugar
• 2 tbsp balsamic vinegar
• 8 oz puff pastry
• 1 egg
• ¼—½ cups shredded gruyere
• 8—10 asparagus stalks, washed and tough ends removed
• Salt and Pepper to taste