Heat canola oil in a large skillet over high heat.
Add onion, garlic, and ginger; cook until fragrant, about 15 seconds.
Stir in shrimp with salt, pepper, and red pepper flakes; cook until slightly pink, 2–4 minutes.
Add beer, ketchup, and sugar; bring to a boil.
Dissolve cornstarch in water and stir into sauce.
Cook until sauce thickens and shrimp are bright pink, about 2 minutes.
Remove from heat, drizzle sesame oil, and garnish with green onions.
