1/4 cup olive oil
1 medium yellow onion, large dice
4 medium Jerusalem artichokes (about 8 ounces), also known as sunchokes, peeled and cut into large dice
2 medium parsnips (about 12 ounces), peeled and cut into large dice
2 medium Yukon Gold potatoes (about 12 ounces), peeled and cut into large dice
1 medium carrot (about 5 ounces), peeled and cut into large dice
1 medium celery root (about 20 ounces), also known as celeriac, trimmed, peeled, and cut into large dice
1 medium fennel bulb (about 12 ounces), stalks trimmed, cored, and cut into large dice
1 medium turnip (about 7 ounces), peeled and cut into large dice
1 tablespoon kosher salt, plus more as needed
6 cups (1 1/2 quarts) low-sodium chicken broth or vegetable broth
1 tablespoon freshly squeezed lemon juice, plus more as needed
1 teaspoon finely chopped fresh rosemary leaves
2 to 3 dried pequín chiles or pinches of red pepper flakes (optional)