How to Make It
In a blender or food processor, combine the shallots, garlic, thyme, parsley, rosemary, mustard and 1/3 cup of the olive oil; blend until smooth. Transfer to a bowl and stir in the butter.
Arrange the lamb fat side down on a rack set over a rimmed baking sheet. Spread the herb butter all over the lamb, leaving a 1-inch border. Refrigerate until the butter is semifirm, 30 minutes. Roll up the lamb with the butter inside and tie securely with kitchen twine at 1-inch intervals. Cover and refrigerate overnight.
Preheat the oven to 400°. Rub the lamb with 2 tablespoons of the oil and season with salt and pepper. Roast for about 1 hour and 45 minutes, until an instant-read thermometer inserted in the thickest part registers 130° for medium-rare. Transfer the lamb to a cutting board and let rest for 20 minutes. Leave the oven on.
On a rimmed baking sheet, toss the tomatoes with the remaining 2 tablespoons of olive oil and the thyme sprigs and season with salt and pepper. Roast for 20 minutes, stirring, until softened.
Discard the strings and thinly slice the lamb. Serve with the roasted tomatoes and olive butter.