In a 2-quart microwave-safe casserole micro-cook the sliced potatoes, water, and salt, covered, on 100 percent power (high) for 7 to 10 minutes or until potatoes are just tender, stirring twice. Drain; remove from casserole and set aside.
In same casserole cook onion in margarine or butter, covered, on high for 1-1/2 to 3 minutes or until tender. Stir in flour. Add milk all at once; stir to combine. Cook, uncovered, on high for 5 to 6 minutes or until thickened and bubbly, stirring after every minute until sauce starts to thicken, then after every 30 seconds. Add cheese; stir until melted. Stir in the cooked potatoes, spinach, and pimiento. Cook, covered, on high for 1 to 2 minutes or just until heated through. Makes 8 servings.