In a large bowl, mix together the olive oil, lemon juice and garlic. Add the lamb chunks, season with ground pepper and stir to coat. Cover the bowl and refrigerate for a minimum of 4 hours.
After the meat has marinated, set sous vide machine to 57.5C/136F.
Place lamb in freezer bag with the marinade (See Note 1). Remove the air through a vacuum sealer or the displacement method. Drop the bag in the bath for an hour.
Remove bag from bath. Take lamb out of the bag, pat dry, season with oregano, onion powder, and salt and pepper to taste. Thread the lamb chunks onto to skewers (See Note 2). Set the bag containing the liquids to the side.
Turn grill on high and let the grill heat up for 5 minutes. Place skewers on grill, brush the lamb with the marinade from the bag and grill for 2 minutes. Flip the skewers over, brush the lamb with the marinade and grill for 2 more minutes. Remove skewers from grill and let rest for 5 minutes. Serve with fresh tzatziki, pita, sliced red onions, or enjoy as is!
Alternative instruction - although grilling is recommended, you can also finish the lamb on a pan. Heat medium-size pan on high and add 1 Tbsp of olive oil. Sear for 1 minute on each side, brushing the lamb with the marinade while they cook. Remove from pan, let rest for 5 minutes, and enjoy!
Peel cucumber and roughly chop. Sprinkle them with salt, set in colander and place a heavy item on top to draw out water. Let sit for 30 minutes, then drain well and wipe dry with paper towels.
Add cucumbers, garlic, lemon juice, dill and a sprinkle of pepper in a blender or food processor. Blend the mixture until smooth, then stir in Greek yogurt. Add in salt to taste. Place tzatziki in refrigerator for at least one hour to ensure the flavors are blended.