Using a mortar and pestle or small spice grinder, combine juniper berries, rosemary and black peppercorns. Mix or grind until ground to a dust-like mixture. Stir in salt. Rub beef all over with spice mixture. Pressing to adhere to the roast. Makes 2 tbsp (30 mL) spice dust.
Heat oil over medium-high heat in heavy skillet; add beef and cook, turning until golden brown all over, about 5 minutes. Place pan with roast and the garlic cloves in preheated 350˚F (180˚C) oven for 15 to 20 minutes for medium-rare doneness or until digital instant-read thermometer inserted into thickest part of beef reads 145˚F (63˚C).
Meanwhile, make peas: heat oil over medium heat, add onions and garlic; cook, stirring, until onions are softened, about 5 minutes. Add peas and chicken broth. Cover and cook until peas are softened, about 5 minutes. Uncover and cook to reduce any liquid, about 3 minutes. Season with salt and pepper. Purée in blender to use as a garnish for the plate.
Remove beef roast and place on cutting board. Cover with foil and let stand for at least 15 minutes. Thinly slice beef.
Meanwhile, if desired, make Garlic Beef Sauce: squeeze out roasted garlic cloves. Add butter to any pan drippings in heavy skillet over medium-high heat. Whisk in flour, cook, stirring constantly for 1 minute. Gradually whisk beef broth into roasted garlic. Cook, stirring up any browned bits from bottom of pan. Whisk until sauce thickens, about 5 minutes.
To serve as a composed starter course, use salad size plates, and place 3 dollops of pea puree on each plate. Top each dollop with a slice of beef and drizzle with garlic beef sauce (if using).