Preheat oven to 350°.
To prepare croutons, place the bread cubes on a baking sheet; coat with cooking spray. Bake at 350° for 12 minutes or until lightly browned and crisp, stirring once. Cool completely.
To prepare dressing, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add shallots, cumin, and coriander; sauté 3 minutes or until shallots are tender. Stir in 2 tablespoons orange juice and vinegar; cook 2 minutes or until liquid evaporates, stirring constantly. Remove from heat; cool slightly.
Combine remaining 2 tablespoons of orange juice, buttermilk, and next 5 ingredients (through pepper), stirring with a whisk. Stir in shallot mixture.
To prepare salad, combine lettuce, fennel bulb, and radicchio in a large bowl. Drizzle with dressing; toss gently to combine. Arrange about 1 1/4 cups salad over each of 8 salad plates; top evenly with orange sections and onion. Sprinkle evenly with croutons and fennel fronds, if desired.