1. Heat the butter in a large stock pot until the foam subsides but without coloring it, then add the shallots and salt.
2. Sweat (do not brown or caramelize) the shallots over moderate heat.
3. When the shallots become soft and translucent, turn off the heat, add the tinned Romas, stir the pot, crush them with your hand, bring the mixture to a bare simmer, and cook it very gently for five minutes only.
5. Correct the seasoning, portion it in zip top bags, and freeze. Count on a half-cup of sauce per adult serving. Rather than labeling each bag, you can keep them in a labeled plastic container which can be stacked conveniently among others containing frozen liquid ingredients such as stocks and broths.