Lentil and Swiss Chard Soup

INGREDIENTS

• 1 tablespoon olive oil, plus more for serving
• 1 medium onion, finely chopped (1 cup)
• 1 tablespoon tomato paste
• 1 teaspoon dried oregano
• 1 teaspoon dried thyme
• 1 cup lentils, picked over and rinsed
• 1 can (14.5 ounces) diced tomatoes, in juice
• 2 bunches (about 1 1/2 pounds total) Swiss chard, stalks cut crosswise into 1-inch pieces, leaves torn into 2-inch pieces (keep stalks and leaves separate)
• Coarse salt and ground pepper
• Juice of 1/2 lemon
• Bread, for serving (optional)