Whisk pineapple juice, hoisin sauce, soy sauce, sherry, and cornstarch together in a small bowl until well combined; set aside.

Drizzle vegetable and sesame oils around the top of a wok or large skillet to coat the sides. Heat over medium-high heat for 1 minute. Pat shrimp dry with a paper towel and add to wok. Stir-fry for 1 minute on each side and transfer cooked shrimp to a clean plate.

Add pineapple, red bell pepper, onion, and snow peas to the wok, and stir-fry for 2 to 3 minutes. Stir in garlic and red pepper flakes, and cook until fragrant, about 30 seconds.

Return shrimp to the wok, and stir-fry an additional 2 minutes. Whisk stir-fry sauce a couple of times and pour over shrimp and vegetables, stirring constantly until thickened, about 1 minute.

Serve immediately, garnished with scallions and sesame seeds.