Preheat the oven to 425 degrees F.

Wrap the red beets in 1 large piece of aluminum foil, and the golden beets in another large piece of aluminum foil, closing the packets tightly. Roast in the oven until tender, about 1 hour. Remove from the oven, unwrap the beets and cool. When the beets are cool enough to handle, peel and cut into 1/2-inch cubes. Place the beets in a small bowl.

Spread the sunflower seeds on a small baking sheet and toast lightly in the oven, about 3 minutes, shaking once during cooking to turn. Remove from the oven and cool.

Place the fennel and mache in a large bowl.

In a small bowl, mix together the shallots, lemon juice, salt, and pepper, and whisk in the oil in a steady stream to emulsify. Toss the beets with a small amount of the dressing, and the fennel and mache with the remaining dressing. Adjust seasoning to taste with salt and pepper.

Divide the mache and fennel among 4 salad plates. Arrange the beets on top of the greens, and top with the crumbled cheese. Sprinkle 2 tablespoons of seeds over each salad and serve with toasted French bread croutons.