Bring a saucepan of salted water, large enough to hold the cabbage, to a boil. Add the cabbage to the water, lower heat to simmer, and cook for 5 minutes. Chill the cabbage in an ice bath. Drain well. Cut crosswise into 3-mm wide shreds. Set aside.
Heat a grill pan until it is very hot, or prepare a barbecue grill. Lightly brush both sides of the fillets with grapeseed oil and season them with salt and pepper. Grill the fillets until barely cooked. Set aside and keep warm.
In the meantime heat a couple of tablespoons of olive oil in a wide frying pan over medium heat. Add the bacon and cook for awhile. Do not let the bacon become crisp. Add the shredded cabbage, season with salt and pepper, and mix well. Cover the pan and cook the combination for a few minutes. Stir periodically to keep the cabbage from burning.
When the cabbage is heated through, lightly salt the diced tomato and add it to the cabbage mixture. Combine well and remove the pan from the heat.
Mound the cabbage on individual serving plates and arrange the fish fillets on top of the cabbage.