Balsamic Green Beans with Red Onions and Shallots

olive oil 1/2 red onion, sliced thinly 1 shallot, sliced thinly 3 cloves of garlic, minced 1/2 cup of beef broth 2 cups of green beans, trimmed 2 tbsp of balsamic vinegar Sea salt and freshly cracked pepper, to taste 1 tsp butter