Season the fillet of ostrich with salt and pepper; wrap it in cling film and place in the freezer for 2 hours.
Place the pecorino, mascarpone and lemon zest in a food processor and chop to mousse-like consistency.
Gently cook the chopped garlic in olive oil for 2 minutes. Blanch the broccoli in boiling salted water for 3 minutes then drain and add to the garlic; cook on medium heat for 3-5 minutes. Pass them in the food processor and process to fine puree. Season and set aside.
Pan roast the beetroot in a nonstick pan with a little olive oil for 2-3 minutes.
To serve, finely slice the ostrich fillet and place on the cheese mousse.