Follow these steps for a perfect Shrimp and Pineapple Stir-Fry every time:

1. Prepare the Shrimp: If using frozen shrimp, ensure they are fully thawed. Pat the shrimp dry with paper towels. In a medium bowl, toss the shrimp with 1 tablespoon of cornstarch until lightly coated. Set aside. This step helps the shrimp get a nice sear and helps the sauce cling to it.
2. Mix the Sauce: In a small bowl, whisk together all the sauce ingredients: low-sodium soy sauce, rice vinegar, brown sugar, sesame oil, and the optional pineapple juice. In a separate tiny bowl or cup, mix the 1 teaspoon of cornstarch with 1 tablespoon of water to create a slurry. Set both aside.
3. Prep the Aromatics and Vegetables: Ensure all your vegetables (bell peppers, red onion) are sliced, garlic is minced, and ginger is grated. Having everything prepped (mise en place) is crucial for a successful stir-fry, as the cooking process is quick.
4. Heat the Pan: Place a large skillet or wok over medium-high heat. Add the olive oil or avocado oil and let it get hot until it shimmers.
5. Cook the Shrimp: Add the cornstarch-coated shrimp to the hot pan in a single layer. Be careful not to overcrowd the pan; cook in batches if necessary. Cook for 1-2 minutes per side, until pink and just cooked through. Remove the shrimp from the pan and set aside on a plate. Don’t overcook, as they will cook a bit more when returned to the sauce.
6. Sauté Aromatics and Vegetables: Add the sliced red onion and bell peppers to the same skillet. Stir-fry for 3-4 minutes until they are tender-crisp. Add the minced garlic and grated ginger (and red pepper flakes, if using) to the pan and stir-fry for another minute until fragrant. Be careful not to burn the garlic.
7. Add Pineapple: Add the fresh pineapple chunks to the skillet. Stir-fry for 2-3 minutes, allowing the pineapple to heat through and slightly caramelize on the edges. This brings out its sweetness.
8. Combine and Sauce: Return the cooked shrimp to the skillet with the vegetables and pineapple. Give the prepared stir-fry sauce a quick whisk (as the sugar might have settled) and pour it over everything in the pan. Stir well to coat all ingredients.
9. Thicken the Sauce: Bring the sauce to a simmer. Add the cornstarch slurry (cornstarch mixed with water) to the pan, stirring constantly. Cook for another 1-2 minutes, or until the sauce has thickened to your desired consistency and coats the shrimp and vegetables nicely.
10. Serve Immediately: Remove from heat. Taste and adjust seasoning if necessary (you might want a tiny bit more soy sauce or a pinch of sugar). Serve hot.