Rinse the beef and pat dry. Cut into 3/4-inch (2x2 cm) cubes. Marinate the beef with 1 teaspoon minced garlic, 1/2 tablespoon fish sauce, 1 tablespoon sweetened condensed milk, and 1 tablespoon cornstarch. Mix well. Remove seeds from the bell peppers and cut into cubes. Cut the onion into cubes. Slice the carrot thinly, shaping as desired. Wash the celery and leek, then cut into pieces.

Sauté garlic until fragrant. Add the bell peppers, onion, and carrot. Stir-fry over high heat with a pinch of salt until just tender, then transfer to a plate. Add the beef to the pan and stir-fry over high heat for about 5 minutes, until just cooked through. Transfer the beef to a separate plate.

Add the sauce ingredients to the pan and stir until combined. Return the vegetables and beef to the pan along with the celery and leek. Stir-fry together for 1-2 minutes until everything is well coated.