sweet potato and sausage soup

INGREDIENTS

• 3 tablespoons extra-virgin olive oil, divided
• 1 10- to 11-ounce fully cooked smoked Portuguese linguica sausage or chorizo sausage, cut crosswise into 1/4-inch-thick slices (Spanish chorizo can be substituted)
• 2 medium onions, chopped
• 2 large garlic cloves, minced
• 2 pounds red-skinned sweet potatoes (yams; about 2 large), peeled, quartered lengthwise, cut crosswise into 1/4-inch-thick slices
• 1 pound white-skinned potatoes, peeled, halved lengthwise, cut crosswise into 1/4-inch-thick slices
• 6 cups low-salt chicken broth
• 1 9-ounce bag fresh spinach