Sear the Beef: Heat 1 tablespoon oil in a large non-stick skillet (or wok) over medium-high heat. Season beef with salt and pepper; Sear the beef for about 2 minutes, turning occasionally, until it's browned on all sides but not fully cooked. Remove and set aside.
Sauté the Veggies: Add remaining oil to skillet. Sauté onion and red pepper for 4 minutes until softened. Add garlic and sauté 1 more minute.
Build the Curry: Stir in red curry paste until fragrant, about 1 minute. Add coconut milk, fish sauce, and sugar. Bring to a simmer.
Finish and Serve: Add zucchini and simmer for 3 minutes. Return beef to skillet; cook for 2 minutes until warmed through.
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