For grilled fillet of beef with bitter orange BBQ sauce, first peel the onion, garlic and ginger for the BBQ sauce and blend in a food processor with the chili pepper to a fine paste. Sauté in a saucepan with the spices, herbs and sugar in a little olive oil for 5 minutes, then deglaze with the bitter orange juice and allow to reduce slightly. Add the bitter orange zest, ketchup, mustard, Worcester sauce and paprika powder and simmer over low heat for about 30 minutes. Then strain the sauce through a sieve and season to taste with salt and pepper.
Prepare the mini potato gratin with mountain cheese according to this recipe.
Remove the beef fillets from the refrigerator an hour before grilling them. Brush with a little olive oil and sear on the grill or in a pan on both sides for about 2 to 3 minutes. Then let rest briefly and season with coarse sea salt and pepper.
Just before serving, mash the potato gratin with a fork, season with a little olive oil and salt and sprinkle with fresh herbs.
Arrange the grilled fillet of beef with bitter orange BBQ sauce with the mini potato gratin and serve.