Preheat the oven to 375 degrees . On a baking sheet, toss the carrots with 1 tablespoon olive oil and season with salt and pepper. Roast until tender, 25 to 30 minutes.
Meanwhile, using a food processor, pulse the toast to make breadcrumbs. Transfer to a medium bowl. Pulse the cashews until finely ground. Toss with the breadcrumbs and parmesan; season with pepper.
Spread the mustard evenly over the tilapia to coat; season with salt and pepper. Sprinkle each fillet evenly with a generous amount of the breadcrumb mixture and press to adhere. Drizzle 2 tablespoons olive oil on top. Transfer to a baking sheet and roast in the oven until the flesh is opaque, about 12 minutes.
In a medium skillet, heat the remaining 1 tablespoon olive oil over medium heat. Add the garlic and cook until fragrant, about 30 seconds. Add the spinach and cook, tossing occasionally, until it just starts to wilt, about 1 minute; season with salt and pepper. Serve with the fish and carrots.