Baked Eggs with Potatoes, Spinach and Cheese Sauce

In saucepan of boiling salted water, cover and cook potatoes until tender, about 20 minutes; drain well and return to pot. Mash with buttermilk, butter, salt and pepper until smooth. Spread in greased 13- x 9-inch (3 L) glass baking dish.

Spinach Layer: Meanwhile, rinse spinach; shake off excess water. In large saucepan or Dutch oven, cover and steam spinach over medium-high heat, with just the water clinging to leaves, stirring once, until wilted, about 5 minutes. In sieve, press out liquid; let cool enough to coarsely chop.

In skillet, heat oil over medium heat; fry onion until softened, about 4 minutes. Add spinach, salt and pepper; stir to combine. Spread over mashed potatoes. (Make-ahead: Cover and refrigerate for up to 12 hours.)

Cheese Sauce: In saucepan, melt butter over medium heat. Add flour; cook, stirring, for 1 minute. Gradually whisk in milk; cook, whisking, until thickened to consistency of whipping cream, about 10 minutes.

Stir mustard, salt and pepper into sauce. Add cheese; stir until melted. (Make-ahead: Place plastic wrap directly on surface; refrigerate for up to 12 hours. Rewarm to continue.)

Thinly slice eggs and arrange, slightly overlapping, over spinach. Cover with cheese sauce.

Bake in 350°F (180°C) oven until edges are golden, about 30 minutes.