Place the stock and lentils in a medium saucepan. Cover and simmer until the lentils are soft and most of the liquid will has been absorbed, about 25-30 minutes.
While the lentils are simmering heat 2 tablespoon of the olive oil over med.- high heat in a large skillet or frying pan . Add the onion and cook until soft and beginning to brown around the edges, about 5 minutes. Add the red pepper and cook for another 4-5 minutes. Add the minced garlic and cook for another minute.Transfer to a plate or bowl and set aside while you cook the eggplant.
Add the remaining 2 tablespoons of olive oil to the pan, and increase the heat to high. add the cubed eggplant and cook until , tossing or stirring frequently, until lightly browned.
Reduce the heat to medium and add the tomato paste and the remaining tablespoon of olive oil and cook for another 2 minutes, stirring constantly.
Next, add the onion, pepper mix back to the pan and stir in the chopped rosemary, cinnamon and ground cumin.
Add the wine, chopped tomatoes and the cooked lentils to the pan and stir to combine everything together. Season with salt and pepper and let simmer for another 5 minutes.
Prepare the Topping
While the filling for the moussaka is simmering, prepare the topping.In a medium size saucepan, melt the butter over medium heat. Add the flour and cook, stirring constantly for 1-2 minutes. add the warm milk and the freshly grated nutmeg. Whisk until it comes to a simmer and begins to thicken into a sauce, about 2-3 minutes.Remove from the heat and allow to cool slightly before whisking in the ricotta and parmesan cheese. Gently beat the egg, then add to the sauce. whisk to combine.
Assemble and Bake the Moussaka
Transfer the vegetable lentil mixture to a baking dish and spoon the sauce evenly overtop.Bake the moussaka on the middle shelf of the preheated oven for 1 hour. allow to rest for 5-10 minutes before serving.