Core and seed peppers. Slice into 1/2-inch strips.
In a small bowl, stir cornstarch with soy sauce until cornstarch dissolves. Stir in sugar and sesame oil.
Heat vegetable oil in a large non-stick frying pan set over medium-high heat. Add scallops and stir-fry until golden brown on both sides, from 1 to 2 minutes per side. Remove to a plate.
Add peppers, vinegar, garlic, ginger and chili flakes to pan. Stir-fry until peppers are tender-crisp, from 2 to 3 minutes. Return scallops and any accumulated juice to pan. Stir soy mixture, then pour into pan. Stir constantly until liquid reduces to a glaze, about 1 minute.
Stir in spinach just until wilted, about 1 minute.