- 1 1/2 pounds fresh green beans, trimmed
- 4 cups sliced onions
- 2 tablespoons balsamic vinegar, or more if needed
- 3 cloves garlic, chopped
- 2 teaspoons white sugar
- 1 teaspoon dried basil
- 1 teaspoon dried oregano
- 1/4 cup shredded Parmesan cheese, or to taste
- halved grape tomatoes
- Bring a large pot of lightly salted water to a boil; cook green beans at a boil until tender yet firm to the bite, 5 to 10 minutes; drain and transfer to a 9×13-inch dish.
- Cook and stir onions, vinegar, garlic, sugar, basil, and oregano in a skillet over medium heat until onions are softened and translucent, about 5 minutes. Reduce heat to medium-low, and continue cooking and stirring until onions are very tender and dark brown, 15 to 20 minutes more.
- Preheat oven to 400 degrees F (200 degrees C).
- Spread onion mixture over green beans and top with Parmesan cheese. Arrange tomatoes, cut sides down, atop Parmesan cheese layer.
- Bake in the preheated oven until cheese is melted and bubbling, about 35 minutes.