Wash the potatoes thoroughly and cook in plenty of boiling water for about 20 minutes. Wash the kale thoroughly and drain. Strip leaves from the stems and pluck or cut into bite-sized pieces.
Blanch the kale in boiling salted water for about 5 minutes. Pour onto a sieve, quench under cold water and let cool off. Pour the potatoes also on a sieve, rinse with cold water and remove the skin.
Let the potatoes cool down a little, then cut them into slices. Peel and finely chop the onion. Roast sunflower seeds in a pan without fat and let them cool down. Cut the bacon slices into thirds. Heat 1 tablespoon of oil in a frying pan and leave the bacon crisp.
Put bacon on a plate, remove pan from heat. Put onion cubes, grapefruit juice, vinegar and remaining oil in the hot pan and season with salt and pepper. Stir well and mix with potatoes and kale.
Let it soak for about 15 minutes, then season again with salt and pepper and possibly vinegar. Meanwhile peel the grapefruit so that the white skin is completely removed. Cut the fruit into slices.
Line a round plate at the edge with the grapefruit slices and place the salad in the middle. Serve sprinkled with sunflower seeds.