Lemon-herb Lamb Roast

• 325 def oven
• 3 center leg (boneless cut)
• 1 deep slits in the surface
• 700 ml yogurt (plain)
• 16 ml lemon juice
• 8 gr cornstarch
• 4 gr beef boullion granules
• 1 gr lemon peel (finely shredded)
• 1 gr rosemary (dried)
• 1 gr thyme (dried crushed)