Cut pork across grain into thin slices; coat with mixture of 1 Tbsp. each soy sauce and cornstarch, 1/2 teaspoon sugar and garlic. Let stand 15 minutes. Blend remaining 2 Tbsp. soy sauce, 1 Tbsp. cornstarch, 1/2 teaspoon sugar, fennel and 3/4 cup water.
Heat 1 Tbsp. oil in hot wok or large skillet over high heat. Add pork and stir-fry 2 minutes; remove. Heat remaining 1 Tbsp. oil in same pan. Add onion and whites of green onions; stir-fry 1 minute. Add carrots; stir-fry 3 minutes. Add green onion tops; stir-fry 1 minute. Add pork and soy sauce mixture. Cook, stirring, until sauce boils and thickens. Sprinkle with cashews; serve over rice.