Separately, in a medium-sized skillet add the olive oil and butter; place over medium-high heat and let the butter completely melt before adding the sliced onion. Sauté for one minute then stir-in the chopped garlic. Cook for about 30 seconds and incorporate the shrimp. Stir occasionally and cook until the shrimp have begun to turn orange in color. When ready, pour the tomatillo salsa over the shrimp and simmer until the sauce begins to boil.