Place the lentils, onion, carrots, celery, potatoes, diced tomatoes, vegetable broth, thyme, smoked paprika, salt, and pepper in the slow cooker. Stir to combine.
Cover and cook on low for 7 to 8 hours until the lentils are soft and the vegetables are tender.
About 30 minutes before serving, preheat the oven to 400°F. In a mixing bowl, whisk together the cornmeal, flour, baking powder, sugar, and ½ teaspoon salt.
Add the milk, egg, and melted butter to the dry ingredients. Stir until just combined.
Pour the batter into a greased 8-inch baking pan. Bake for 20 minutes until the top is golden and a toothpick comes out clean.
Serve scoops of stew alongside wedges of warm cornbread.