Place potatoes in microwave-safe 8 x 8 x 2-inch baking dish. Add 1/4 cup water; cover with microwave-safe plastic wrap, venting slightly at one corner. Microwave on high 10 minutes; rotate halfway through cooking. Let stand, covered, for 10 minutes.
Combine half-and-half, butter, salt, pepper, nutmeg, and garlic powder in microwave-safe measuring cup. Microwave, uncovered, on high 1 to 1-1/2 minutes, until steaming.
Mash potatoes. Add half-and-half mixture; cover with microwave-safe plastic-wrap; keep warm. Microwave, covered, on high 1 to 2 minutes if necessary to heat before serving.
Combine oil, tomatoes, garlic, tomato sauce, salt, Italian seasoning, onion powder and pepper flakes in microwave-safe 13 x 9 x 2-inch dish. Microwave, uncovered, on high 5 minutes.
Add olives; stir. Add tuna; turn to coat. Spoon sauce over steaks. Cover with microwave-safe plastic wrap, venting slightly at one corner. Microwave on high 4 minutes.
Rotate pan. Uncover. Turn fish over. Spoon sauce over top. Recover, venting. Microwave on high 4 minutes. Let stand, covered, 10 minutes. Serve with mashed potatoes and lemon.
In a pot, add water to potatoes to cover by 1 inch. Cover; cook until tender, 12 to 15 minutes. Drain. Return potatoes to pot; mash. Heat half-and-half, butter, salt, pepper, nutmeg and garlic powder in saucepan over medium-low until steaming; stir into potatoes. Cover with foil. Meanwhile, heat oil in large skillet over medium-high heat. Add tomatoes and garlic; sauté 3 minutes. Add tomato sauce, salt, Italian seasoning, onion powder and pepper flakes; cook 5 minutes. Stir in olives. Add tuna, turn to coat. Spoon sauce over tuna. Cook, covered, over medium heat 10 minutes or until fish flakes, turning over halfway through; baste occasionally. Serve with mashed potatoes and lemon.