In a blender or food processor, process the salsa to a smooth puree.
Heat the oil in a very large (12 inch) skillet over medium high heat. When it is hot, add the salsa all at once. Stir as the salsa reduces to the consistency of tomato paste, about 5 minutes.
Stir in the broth and the tahini. Return to a boil, then reduce the heat to medium low and let simmer 10 minutes. Taste and season with salt, usually about ½ tsp, and a little sugar. (The sugar will help balance the natural tartness of the salsa.).
While the sauce is simmering, pour the peas into a microwaveable bowl, sprinkle on a Tbsp of water, cover with plastic wrap and poke a couple of holes in the top. Microwave on high until the peas are hot and tender, anywhere from 1 minute for frozen peas to 4 or 5 minutes for fresh peas; discard water.
When the sauce has simmered for 10 minutes, nestle the fish fillets in it, completely submerging them. Continue simmering gently until the fish flakes when pressed firmly, usually 5 to 6 minutes for ½ inch thick fillets. (Check it by lifting up a fillet on a metal spatula and pressing it with your finger or the back of a spoon.).
Transfer a fish fillet to each dinner plate. Spoon a portion of the sauce over the top. Strew with the peas, sesame seeds and cilantro.
You can replace the peas with a couple of medium-large red-skin boiling potatoes cut into eighths (microwave them until tender, about 8 minutes). Mix the potatoes into the sauce after transferring the fillets to the dinner plates. A can of white beans makes a great replacement for the peas; drain and rinse them before adding them to the sauce. This dish is also wonderful made with boneless, skinless chicken breasts or semi-boneless quail; poach the birds in the sauce as described or, for added flavor, brown them in oil in the large skillet, then remove them and, without washing the skillet, cook down the pureed salsa. Tahini is an easy addition because it’s smoothly ground, but you can use the very traditional pumpkin seeds or almonds or peanuts; puree them with the salsa, but stir carefully as you cook the mixture down to a paste (it will stick more easily than the salsa alone) After the sauce has simmered 10 minutes, it will likely be quite coarse looking; reblend the hot sauce in a loosely covered blender to smooth it out.