To prepare onions, combine vinegar, sugar, and dash of red pepper in a microwave-safe bowl. Microwave at high 25 seconds; stir until sugar dissolves. Stir in 1/2 cup onion; cover and refrigerate overnight.
To prepare peas, heat oil in a medium saucepan over medium-high heat. Add 1 cup onion and garlic; cook 2 minutes, stirring frequently. Stir in peas and remaining ingredients, and bring to a boil. Cover, reduce heat, and simmer 20 minutes. Partially mash peas with a potato masher; cook, uncovered, for 5 minutes. Serve topped with pickled onions.