Indian Sweet Potato and Lentil Soup

INGREDIENTS

• 1 tablespoon olive oil
• 1 large onion, minced
• 6 cloves garlic, minced
• 2 pinches red pepper flakes
• 1/2 teaspoon ground turmeric
• 1 1/2 teaspoons garam masala
• 1 cup lentils
• 4 cups vegetable broth
• 2 sweet potatoes, peeled and cut into 1/2-inch cubes
• 4 cups chopped fresh spinach
• salt to taste (optional)