Cut corn kernels from ears of corn; set aside. Reserve cobs.
Heat oil in a large saucepan over medium-high heat. Add onion to pan; sauté 5 minutes or until tender. Cut each cob into 3 pieces. Add cobs, 4 cups water, and next 6 ingredients (through garlic) to pan; bring to a boil. Cover, reduce heat, and simmer 30 minutes. Strain broth through a colander into a bowl; discard solids. Add corn kernels, juice, and salt to broth; stir to blend. Return soup to pan; simmer 5 minutes or until hot. Garnish with lime and jalapeño slices, if desired.