Blushing rhubarb bisque with Challah croutons

Name: Blushing rhubarb bisque with Challah croutons
Ingredients: For the rhubarb bisque; 6 stalks of fresh rhubarb, cut into 2-inch pieces; 1/2 cup water; 1/2 cup turbinado sugar; 6 ounces canned coconut milk (not lowfat); 1 tablespoon agave syrup (or granulated sugar); 1 tablespoon calvados; 1 teaspoon vanilla; Croutons (for 2 servings); 1 piece thick slice of raisin challah (or other raisin or egg bread), crust trimmed off and cut into 1- inch squares; 1/2 tablespoon butter; 2 teaspoons toasted walnut oil; 1/8 teaspoon sea salt (especially if butter is unsalted)