Heat oil in large saucepan over medium heat. Add onions and cook 3 min., until beginning to soften. Mix in carrots, celery, split peas, salt and pepper. Stir in broth. Bring just to a boil over high heat then reduce to a simmer. Cook, partially covered, 30 to 40 min., or until peas and vegetables are tender.
Use an immersion blender to purée soup until smooth. Stir in ham, lemon juice and thyme. Heat soup on low heat, 3 to 4 min., or until ham is warmed through. If soup is too thick, stir in an extra ½ cup (125 mL) to 1 cup (250 mL) water. Serve topped with croutons, if desired.
For a spicier soup, stir in a seeded and finely diced jalapeño pepper when adding carrots.