3 Tablespoons balsamic vinegar
2 Tablespoons olive oil
1/2 cup finely chopped onion
1 Tablespoon fresh thyme leaves or 1 1/2 teaspoons dried thyme leaves
3/4 teaspoon salt
1/2 teaspoon fresh ground pepper
1 1/2 pounds beef top sirloin steak, cut into 1” chunks
1 green bell pepper, core and seeds removed, cut into large chunks
1 red bell pepper, core and seeds removed, cut into large chunks
1 yellow squash, sliced into 1/2” slices