Caldo Verde (Portuguese Sausage Kale Soup)

Ingredients

• 12 ounces linguica sausage, sliced
• 1 tablespoon olive oil
• 1 onion, finely diced
• 1 pinch salt
• 3 pounds russet potatoes, peeled and sliced 1/8 to 1/4 inch thick
• 2 teaspoons salt, plus more as needed
• 2 quarts chicken broth or water
• 2 pounds kale - trimmed, chopped, rinsed, drained
• 1 pinch cayenne pepper