Charred Burgers With Red Onion Jam

Ingredients

• 1/4 cup olive oil
• 2 red onions, thinly sliced
• 1/2 cup sugar
• 1/4 cup red-wine vinegar
• 1/4 cup dry red wine, preferably syrah or zinfandel
• 2 pound ground sirloin and chuck
• kosher salt and freshly ground black pepper, to taste
• 4 slices cheddar cheese
• 4 brioche burger buns
• sliced tomatoes