Heat oil in a 4-quart saucepan, add onions, peppers, zucchini and red curry base and cook, stirring, for 3 minutes. Stir in brown sugar and fish sauce; scoop creamy top off coconut milk (about 1 cup) and add to onion mixture with water. (Discard clear liquid in bottom of can or reserve for other use.) Bring to a simmer.
Add sliced pork, return to a simmer over medium-low heat and cook, stirring occasionally, until pork is just cooked through, about 8 minutes. Sprinkle peanuts on top and garnish with cilantro sprigs.