Heat the butter in a large skillet over medium heat
Cook the ground beef until completely browned, all the pink is gone
Add the chopped onion, pepper and garlic to the skillet and cook until soft.
Add the remaining ingredients, cover the skillet with a lid, turn the lid down to low and simmer for 15 minutes.
Meanwhile, pre-heat the oven to 400 °F and lightly spray an 8x8-inch casserole dish with nonstick spray.
Arrange the taco shells in the casserole dish. Spoon the picadillo filling into the taco shells, and sprinkle the shredded cheddar cheese evenly on top.
Bake the tacos for about 15 minutes, until the shells are lightly browned and the cheese is bubbling. Top with a dollop of Greek yogurt (and a few sprigs of parsley or cilantro, if you like.)