Preheat oven to 500 degrees. Pat the fillet dry and season with salt and pepper. In an oiled roasting pan roast fillet in middle of oven until a meat thermometer registers 130 degrees. for medium-rare meat, 20 to 25 minutes, and let cool to room temperature. Fillet may be roasted 2 days ahead and chilled, wrapped well.
To make vinaigrette: Cut top 1/4-inch off each head garlic and wrap heads together in foil. Roast garlic in a preheated 350 degree F oven for 40 to 45 minutes, or until very soft, and let cool. Squeeze roasted garlic from each head (there should be about 1/3 cup) and in a blender or food processor blend together with mustard, vinegars, and salt and pepper to taste until smooth. With motor running add oil in a very thin stream and blend until emulsified. Vinaigrette may be made 4 days ahead and chilled in a tightly sealed jar. Shake vinaigrette well before serving. Slice fillet thin. Arrange fillet, arugula, and tomatoes decoratively on a platter and serve with vinaigrette.
Recipe courtesy of Gourmet Magazine