Preheat oven to 400 degrees. Make the croutons: Coat a rimmed baking sheet with cooking spray. Arrange bread cubes in a single layer on prepared sheet; coat bread with cooking spray. Season with salt and pepper. Toast in oven, stirring occasionally, until golden brown and crisp, 10 to 12 minutes. Transfer to a plate; let cool completely. Croutons can be stored in an airtight container at room temperature up to 4 days.
Squeeze juice of 1 orange into a small nonreactive saucepan. raisins. Bring to a simmer over medium heat. Remove from heat; let stand until raisins are plump, about 10 minutes. Drain raisins, reserving 2 tablespoons juice.
Coat a rimmed baking sheet with cooking spray. Arrange scallions, fennel, and raisins in a single layer on sheet; coat with cooking spray. Roast, flipping mixture once, until vegetables are tender and lightly browned, 12 to 15 minutes. Transfer to a plate; let cool slightly. Cut scallions into 1-inch pieces.
Remove peel and pith from remaining orange. Carve out flesh between membranes (you should have about 3/4 cup segments). Transfer segments to a large bowl. Add scallions, fennel, raisins, arugula, and croutons; gently toss.
Whisk reserved juice, the vinegar, and salt in a small bowl. Gradually add oil, whisking until emulsified.
Drizzle vinaigrette over salad, and gently toss. Divide among 4 serving plates. Season with pepper.