Heat the olive oil in a large skillet over medium-high heat. Add the onion and garlic and cook until soft and golden brown, 4 to 5 minutes. Stir in the tomato paste and red pepper flakes and cook 1 minute. Stir in 1/2 cup water, scraping up any browned bits. Transfer the contents of the skillet to a 6-quart slow cooker.
Add the chickpeas, squash, chard stems (not the leaves), the parmesan rind, if using, 2 teaspoons salt and 7 cups water to the slow cooker. Stir, then cover and cook on low, 8 hours.
Just before serving, lift the lid and stir in the chard leaves; cover and continue cooking 10 more minutes. Season with salt and pepper, and stir to slightly break up the squash. Discard the Parmesan rind, if used. Ladle the stew into bowls; top with the grated parmesan, if desired, and serve with bread and/or lemon wedges.