Freezing the shrimp, onions, peppers, and seasonings in the same ziplock bag not only makes this fajita dish come together faster, it also acts as a marinade, intensifying the flavor. To maximize freezer space, lay the prepared bags flat in a single layer in the freezer; when frozen, stack bags to save room. Before broiling, make sure the mixture is completely thawed with no icy chunks, which will water down the dish.