Heat oil in a large saucepan or wok over medium heat. Add the garlic and ginger, and simmer until aromatic—about 1-2 minutes. Don't brown the garlic.
Add the bell pepper, carrots, and scallions. Sauté until tender—about 2-3 minutes. Add the pineapple chunks and the sweet and sour sauce. Let it simmer for a few minutes as the sauce thickens a bit. If you want a thicker sauce (I always do), mix 1/4 cup of cold water with 1 TBSP of corn starch in a small bowl. Mix until corn starch is completely dissolved, and slowly add it to the simmering liquid, stirring constantly, until the sauce reaches your desired consistency.
Bring the heat to medium-low. Add the shrimp and toss to coat. Shrimp cooks quickly, so you'll only need to let it simmer for about 2 minutes until it's cooked through. Once the shrimp is opaque, it's ready. Don't overcook, or the shrimp will be rubbery.
Remove from heat and toss in the toasted cashews (see notes for toasting instructions).
Serve over rice or quinoa, and garnish with scallions and sesame seeds.