Simmer lentils in water (2 cups) with 1/4 teaspoon salt in a 1- to 1-1 1/2 quart saucepan, uncovered, adding more water if necessary to keep lentils barely covered, until tender but not falling apart, 20 to 25 minutes. Remove from heat and season with salt.
While lentils simmer, heat 1/4 oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then saute onion with pepper and remaining 1/2 teaspoon salt, stirring, 1 minute. Reduce heat to low and cook, covered, stirring occasionally, until onions are soft and golden (stir more frequently toward end of cooking), about 20 minutes. Remove lid and increase heat to moderate, then cook, stirring frequently, until onion is golden brown, 5 to 10 minutes more.
While onion cooks, cut out and discard stems and center ribs from kale. Cook kale in a 6- to 8-quart pot of boiling salted water, uncovered, stirring occasionally, until just tender, 5 to 8 minutes. Transfer kale with tongs to a colander to drain, pressing lightly. Keep pot of water at a boil, covered.
Coarsely cho kale and add to onion along with lentils (including lentil-cooking liquid), then simmer, stirring, 1 minute. Season with salt and pepper.
Add pasta to kale-cooking liquid and boil, uncovered, until al dente. Reserve about 1 cup pasta-cooking liquid, then drain pasta in a colander. Add pasta to lentil mixture along with about 1/3 cup of pasta-cooking liquid (or enough to keep pasta moist) and cook over high heat, tossing, 1 minute. Season with salt and pepper and drizzle with remaining 2 tablespoons oil.
Lentils and onions can be cooked up to 5 days ahead and chilled, uncovered, until completely cooled, then covered. Reheat over low heat, thinning with water as necessary.
Kale can be washed and trimmed 1 day ahead and chilled in a sealed plastic bag lined with dampened paper towels.