Juicy grilled hamburger with caramelized onions, grilled pineapple, topped with melted cheddar cheese, tomato slices and red onion.
First, we need to caramelize the onions, prior to grilling the burgers and the pineapple. Heat 1 tablespoon olive oil on high heat in a large skillet, when oil is heated, add sliced onions and cook on high heat for about 10 minutes, constantly stirring with spatula. The onions should start to brown, but without burning (a couple of onions may be burnt here and there, but overall they should not be charred).
Reduce heat to medium and continue cooking onions for 10 more minutes, continuing to stir, as onions brown even more without burning. At this point add just a pinch of salt over onions.
Continue cooking for 10 more minutes on medium or low heat, stirring occasionally to make sure onions don’t stick to the bottom of the pan or burn.
Total you should have cooked onions for 30 minutes. Remove from heat and sprinkle onions with a small amount of balsamic vinegar to deglaze the pan. Using spatula, mix the onions, scraping the bottom of the pan and coating onions with a pinch of balsamic vinegar you just added.
Brush both sides of each hamburger patty with vegetable oil (otherwise, they will stick to the grill). Grill hamburger patties for about 3 minutes on the first side.
Flip them over, and cook the other side of each hamburger patty for 4 minutes for medium-rare burgers or 6 minutes for well-done burgers.
In the last minute of cooking, arrange a slice or two of cheddar cheese on top of each hamburger patty to melt.
Arrange lettuce on the bottom of each bun.
Place grilled hamburger patties (covered with melted cheddar cheese) on top of lettuce.
Top each burger with caramelized onions, then tomato slices, and red onion slices.
Finally, add grilled pineapple slice(s).
Close with the top bun, if desired, or serve these as open-faced hamburgers.