Red and yellow bell peppers combine with harissa, a Tunisian hot chili sauce, to make a lively, rustic sauce that is especially good with meaty fish steaks, like tuna or swordfish. Or try it with chicken, zucchini, or string beans. If you don't have harissa, there is an easy solution: dried hot pepper flakes. In other words, heat is an integral part of this relish. Don't just leave it out.