Simmer split peas with 2 cups water in a small saucepan until just tender, 25 to 30 minutes. Drain and pat dry on a towel.
Heat oil in a large frying pan over medium heat. Add mustard seeds, cover, and cook until they pop, 1 minute. Stir in cumin; cook a few seconds until sizzling, then stir in split peas, curry leaves, chile, turmeric, and 1/2 tsp. salt. Cook, stirring often, until split peas start to turn light golden, 2 minutes. Scrape into a bowl.
Add spinach to frying pan, increase heat to medium-high, and cook, stirring, until wilted, 1 to 2 minutes. Drain any liquid. Stir in quinoa, split-pea mixture, and 1 1/2 tbsp. lemon juice. Add more lemon juice and salt to taste. Scatter cilantro on top.