Saute onion in butter and oil for 5 minutes. Add peas and stock. Cover and bring to a boil. Reduce heat and simmer for 30 minutes. Mix cornstarch with 1/4 cup water. Add to pot and bring back to a boil. Lower heat, add mint, season with salt and pepper. Simmer for 10 minutes. Add cream if using. Garnish with croutons and serve.