This melt-in-your-mouth mixture of creamy fontina, spinach, shallots, and artichoke hearts rolled into crepes and smothered in a bubbling mustard cheese sauce is a must-make dinner.
Preheat oven to 375°, with rack in upper-middle position. Heat oil in a large skillet over medium-high. Add shallots, artichokes, and 1/2 teaspoon salt. Cook, stirring occasionally, until golden brown in places, 5 to 7 minutes. Add spinach, a handful at a time, until all spinach is wilted and tender and any excess moisture has evaporated; season with salt and pepper and transfer to a bowl to cool.
Meanwhile, melt butter in a small saucepan over medium-high. Whisk in flour and simmer until mixture darkens slightly and smells nutty, about 1 minute. Whisk in milk and bring to a vigorous simmer. Continue to simmer, stirring frequently, until mixture is just thick enough to coat the back of a wooden spoon with a thin film, 1 to 2 minutes; remove from heat. Stir in cheese and mustard until smooth; season with salt and pepper.
Fold 3/4 cup cheese sauce into spinach mixture. Working one at a time, spoon 1/3 cup spinach-cheese mixture into the center of a crepe. Lift edge nearest you up and over filling, then fold in sides and roll up like a burrito to make a 2-by-4-inch rectangular package. Transfer to an 8-by-8-inch baking dish and repeat with remaining crepe and spinach-cheese mixture. Pour remaining cheese sauce evenly over filled crepes in baking dish. Cover dish tightly with parchment-lined foil.
Bake on center rack 15 minutes. Remove parchment-lined foil and switch oven to broil. Continue cooking until cheese sauce bubbles and browns in places and crepes are warmed through, 2 to 3 minutes more. Serve immediately.