Heat oven to 425°F. Slice the eggplants into ½-inch rounds, brush with 3 tbsp oil, salt, and roast on sheet pans 20 minutes until soft.
Meanwhile cook the diced onion in the remaining oil 5 minutes, add the garlic and cinnamon, then the lentils, tomatoes and 2 cups water. Simmer 25 minutes until the lentils are tender and the sauce is thick.
For the topping, whisk the cornstarch into the cold oat milk with the nutmeg and a pinch of salt, then cook over medium heat, whisking, until thick like pudding.
Lower the oven to 375°F. Layer half the eggplant in a 9x9 dish, then all the lentils, then the remaining eggplant, and spread the creamy topping over it.
Bake 25 minutes until bubbling and lightly golden. Rest 10 minutes before cutting. Make it a day ahead for easier slicing.
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