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1
Combine 2 1/2 cups water and lentils in a saucepan; bring to a boil. Reduce heat, cover saucepan, and simmer until all the liquid is absorbed and lentils are tender, 20 to 30 minutes.
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2
Mix 1/3 cup water and ground flax seed together in a bowl; set aside until thickened, at least 10 minutes.
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3
Heat olive oil in a large pot over medium heat; cook and stir onion, mushrooms, celery, and garlic until softened and liquid from mushrooms has evaporated, 5 to 10 minutes.
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4
Preheat oven to 350 degrees F (175 degrees C). Grease a loaf pan.
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5
Transfer cooked lentils to a food processor and pulse until about 75 percent of lentils are mashed. Mix onion-mushroom mixture, oats, flour, basil, black pepper, salt, and onion powder into lentils until well combined. Press lentil mixture into the prepared loaf pan.
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6
Combine ketchup, brown sugar, mustard, and barbeque sauce in a saucepan over medium-low heat; cook and stir until glaze is smooth and brown sugar is dissolved, about 5 minutes. Remove saucepan from heat.
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7
Bake loaf in the preheated oven for 35 minutes. Spread 1/4 cup glaze over lentil loaf; continue baking until cooked through, 10 to 15 minutes more. Serve lentil loaf with remaining glaze on the side.
