1. To make the onions, melt the butter and 2 tablespoons of the olive oil in a large skillet over medium-low heat. Toss in the onions. Cook 5 minutes until they begin to release their juices. Raise the heat to medium-high and continue cooking until the onions are soft and golden, about 7 minutes. Add the remaining tablespoon oil and the salt and increase the heat to high. Cook, stirring only a few times, until the onions are crisp and charred in places, 5 to 10 minutes.
2. For the soup, heat the oil in a large pot over medium heat. Add the onion and cook, stirring, until softened, 5 to 7 minutes. Drop in the ginger, garlic, cinnamon, cumin, allspice and bay leaf; cook until the mixture is fragrant, about 1 minute. Stir in the stock, 3 cups water, the rice and the salt. Bring to a boil over high heat, then immediately reduce heat to medium-low and let the rice simmer 10 minutes.
3. Stir in the red lentils and cook until the rice is tender and the lentils are meltingly soft, about 30 minutes.
4. Stir in the spinach and 2 tablespoons of mint, if using, and let them wilt completely. Taste and add more salt if necessary. If the dish seems flat, squeeze in the juice from one or two lime wedges.
5. Ladle the soup into serving bowls and top each bowl with small handful of crispy onions. Garnish with remaining mint if desired and a sprinkling of flaky sea salt. Serve with lime wedges.
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