Lemony Beef Shanks

• 4 garlic (cloves minced)
• 1 onion thinly (sliced)
• 1 lemon zest (grated)
• 350 gr potatoes scrubbed (red)
• 350 ml beer
• 350 gr beef shank trimmed of fat
• 170 gr tomato paste
• 80 gr mushrooms (fresh white sliced)
• 32 gr quick mixing flour
• 30 ml lemon juice