Mix flour, water, yeast, and salt until a rough dough forms. Knead until smooth, then add oil and knead until elastic, 6–8 minutes. Cover and rise for 1 hour. Fold, shape into a ball, and rise 1 hour more. Divide into two pieces.

Sauté garlic and spinach in a lightly oiled skillet until wilted. Drain well. Melt butter in the skillet, whisk in flour, and cook for 1 minute. Gradually whisk in milk until thick. Season with salt, pepper, and nutmeg. Combine with spinach and cheese.

Preheat the oven to 425°F/220°C.

Roll each dough piece into a 10×7-inch oval. Spread béchamel down the center, pinch edges to seal, and flip seam-side down. Cut a small slit on top. Fill with spinach-cheese mixture.

Bake on a parchment-lined sheet pan until golden, 12–15 minutes. Add an egg yolk to each boat and bake 1–3 minutes more until set. Serve warm.