Peel the grapefruit, cut into thin slices and arrange on two plates.
Cut the fennel bulbs lengthways into thin slices and fry them in hot oil, deglaze with the stock and orange juice. Add the thyme and simmer until the fennel is cooked through but still firm to the bite. Let cool slightly.
For the dressing, rub the zest of the lemon and lime and squeeze out 2 tablespoons of juice each. Whisk all ingredients with the lemon and lime juice. Mix in the two rubies.
Place the still lukewarm fennel on the grapefruit slices and drizzle the dressing over them.