Place carrots on a sheet pan. Spread fennel and onion mixed together on another. Drizzle each pan with 1 Tbsp olive oil, seasoning with salt and pepper.
Roast, tossing occasionally, until golden brown and tender, about 20 to 25 minutes for carrots and 35 to 45 minutes for onions and fennel. Set aside and let cool.
In a small sauté pan, heat remaining olive oil and cook sunflower seeds, coriander seeds, red pepper flakes, and paprika over medium heat, stirring often, until oil is gently bubbling around seeds and spices are fragrant, about two minutes. Let cool.
Stir in red wine vinegar and lemon juice and season with salt and pepper. Toss with roasted veggies and garnish with mint.