Step 1

In a Dutch oven or 5-quart heavy pot with a lid, heat oil over medium. Add onion, carrots, celery, and thyme; season with salt and pepper. Cook until vegetables begin to soften, 5 to 8 minutes.

Step 2

Add broth, split peas, ham bone, and 5 cups water. Bring to a boil, reduce heat to medium-low, and partially cover; simmer until peas are soft, 30 to 45 minutes.

Step 3

Meanwhile, make croutons: In a large skillet, melt butter over medium heat. Add bread and cook, tossing occasionally, until browned and crisp, 6 to 8 minutes. Transfer to a paper-towel-lined plate.

Step 4

Remove and discard bone from soup. Working in batches, puree only 1/2 the soup in a blender (don't overfill); return to pot. Add ham cubes, and simmer until heated through. If necessary, thin with water. Add salt, pepper, and lemon juice to taste. Serve topped with croutons.