Place the balsamic vinegar, salt, pepper and 1 tsp crushed tarragon in a bowl. Mix well. Gradually add the olive oil and 4 finely cubed strawberries. Set aside. In another bowl, mix the butter and mustard, 2 tsp tarragon and mix well. Set aside.
In an ovenproof dish, place the minced onions with olive oil, rosemary and thyme, and season with salt. Place in the oven, preheated to 160ºC/Gas mark 3, for 30-40 minutes or until tender and just golden brown.
Cut the tenderloins into tournedos. Season with salt and a little tarragon. Fry the garlic puree in a frying pan for 30 seconds, add the tenderloins and cook on both sides until slightly browned. If you like them cooked for longer, finish in the oven. (They should be a little pink inside.) Remove and set aside.
Add a little water to the frying pan and deglaze. Bring to the boil, then reduce the heat. Add the butter, mustard and tarragon mixture. Serve the small tenderloins with rocket and cover with roasted onions, rolled or cubed strawberries and nuts, seasoned with the strawberry-tarragon vinaigrette.