Bistec Encebollado

What you will need

1/4 cup red wine vinegar (paleo diet: red wine ok)

2 Tbsp olive oil

2 tsp Kosher salt (paleo diet: sea salt)

2 tsp oregano

1 Tbsp cumin

2 garlic cloves minced

2 lbs flank steak

2 Tbsp olive oil

2 onions medium, thinly sliced

1/2 cup chicken stock