Using a mandoline, thinly slice zucchini lengthwise into ⅛-inch thick slices and set on paper towels. Place more paper towels on top and gently press out the water as best you can.
Turn oven to broil and line a baking sheet with parchment paper. Lay zucchini slices onto the baking sheet without crowding, and place in oven for 3 minutes. (You might have to do this in a few batches, so as not to overcrowd the tray.) Gently flip zucchini slices and bake the other side for another 2 minutes. (This will help to get more moisture out of the zucchini, which is necessary to prevent the lasagna from being watery.) Remove from oven and let cool slightly, then gently pat out any excess moisture with paper towels.
To make the sauce, combine garlic, tomato sauce, tomato paste, lemon juice, and salt in a saucepan. Bring to a simmer. Pulse lentils in a blender or food processor and add to the saucepan. Cook sauce for 30 minutes, until thick and the liquid is reduced by almost half.
Preheat the oven to 375°F.
In an 8x11-inch casserole dish, spread a thin, even layer of Bolognese on the bottom and layer the zucchini on top to cover. Pour a thin layer of Alfredo sauce over top of zucchini. Then sprinkle with a thin layer of mozzarella. Repeat the process until all your ingredients are used up (ending with the mozzarella).
Cover with foil and bake for 30 minutes. Remove foil and bake for another 20 minutes. Let lasagna sit for 10-15 minutes before serving.