Sort beans for debris, then rinse. In a 5- to 6-quart pan over high heat, bring beans and about 2 quarts water to a boil. Cover, boil for 2 minutes, and remove from heat. Let stand at least 2 and up to 4 hours. Drain beans, rinse, return to pan, and add 2 quarts water. Bring to a simmer over high heat, cover, reduce heat to maintain a simmer, and cook until beans are tender to bite, about 40 minutes. Drain beans, reserving 4 cups cooking liquid (see notes).
Meanwhile, in a large bowl, mix olive oil and garlic. Add onion, parsnips, squash, 1/2 teaspoon salt, and 1/4 teaspoon pepper; mix to coat evenly. Spread mixture in single layers in two 12- by 15-inch baking pans. Bake in a 400° regular or convection oven for 15 minutes. Stir with a wide spatula and bake until vegetables are tender when pierced, 10 to 15 minutes longer.
Rinse kale and tear leaves from center ribs; discard ribs and stems. In a 6- to 8-quart pan over high heat, bring about 2 quarts water to a boil. Add kale leaves and cook, uncovered, until tender to bite, 2 to 3 minutes. Lift out with a slotted spoon and immerse in cold water until cool. Drain and coarsely chop.
In a 6- to 8-quart pan over high heat, combine beans, the 4 cups reserved bean cooking liquid (see notes), roasted vegetables, tomatoes with their juices, and olives. Adjust heat to maintain a simmer and cook for 15 minutes to blend flavors. Add kale and cook to heat, 1 to 2 minutes. Add additional salt and pepper to taste. Mound polenta on dinner plates or in shallow bowls and ladle vegetable ragout over polenta.