Spinach and Potato Lasagna with Béchamel Sauce

Peel potatoes, rinse and cut into thin slices. Cook in boiling broth for 5-6 minutes. Peel carrots and cut into thin slices. With a skimmer, lift potatoes from broth. Blanch carrots in boiling broth for 3-4 minutes. Drain and rinse in cold water, then drain again. Rinse spinach and spin dry. Remove tough stems and cut large leaves into smaller pieces, if needed. 

Peel onions and dice. Heat oil in a large saucepan. Sauté onions until tender. Peel garlic and squeeze through a press into pan, then add spinach and cover pot. Remove from heat. Heat butter in a pan and sprinkle flour over hot butter and let cook briefly and stir. Deglaze pan with milk and whisk in 0.25 liter (approximately 1 cup) broth. Bring to a boil, stir constantly until thickened. Remove from heat.