Shrimp shells may be left on or removed. Devein shrimp and set aside. If using frozen shrimp, rinse under cold water to remove ice crystals. Do not thaw, but pat dry. To prepare ginger, peel and then grate on large size of a box grater. Then finely mince and measure out.
Combine ginger, coconut milk, soy sauce, sesame oil and chili-garlic sauce in a wide saucepan. Bring to a boil, stirring often, over medium-high heat. Boil gently, uncovered and stirring often, until sauce is very thick, from 15 to 20 minutes. Meanwhile, trim snow peas. Slice in half if large. Seed pepper, if using, then slice into matchstick-size pieces.
Reduce sauce until it just covers bottom of pan and measures about 1/2 cup (125 mL). If making ahead, remove sauce from heat and leave, covered, at room temperature for several hours or refrigerate.
Just before serving, add shrimp, snow peas, pepper, if using, and onions to hot sauce. Stir almost constantly until shrimp are hot and bright pink, from 2 to 4 minutes. Serve over rice or rice noodles. Sprinkle with coriander and squeeze a little lime juice overtop