Cook rice according to package directions. Spread on a sheet pan and refrigerate until chilled, at least 30 minutes. If starting with 3 cups of leftover cold rice, you can skip this step and move straight to Step Two.
Set a large skillet or wok over medium-high heat and add 1 tablespoon vegetable oil. When the oil is hot, add the steak pieces and cook, stirring occasionally, until heated through, about 5 minutes. Season to taste with salt and pepper. Transfer to a bowl and set aside.
Heat another 1 tablespoon vegetable oil in the same skillet. Add the carrots, celery, and leeks. Stir-fry for 2 minutes, until slightly softened. Add garlic and ginger and continue cooking for an additional 30 seconds.
Add the remaining 2 tablespoons vegetable oil, then stir in the chilled rice. Break up any clumps and stir-fry for 4–5 minutes until the rice is hot and lightly crisped.
Return the vegan steak to the skillet. Add the snap peas and soy sauce. Stir-fry for 2–3 minutes until the peas are crisp-tender and everything is evenly coated.
Drizzle with sesame oil and give a final toss. Adjust seasoning to taste, sprinkle with scallions, and serve hot.