Mediterranean salsa soup

Ingredients

• 3 garlic cloves , or 2 tsp bottled chopped garlic
• 1 onion
• olive oil
• 796-mL can diced tomatoes , including juice
• 284-mL can undiluted chicken or vegetable broth
• 1/2 cup salsa
• 170-g can tuna , drained (optional)
• 4 cups baby spinach
• 1 cup frozen corn kernels
• 1 tsp granulated sugar
• 1/2 tsp freshly ground black pepper
• 1/2 tsp dried oregano leaves
• feta , optional