1) To a blender, add the oil, vinegar, garlic, oregano, thyme, lemon juice, and dijon mustard, season with salt and pepper, to taste. Blend until thoroughly combined.
2) Pat the lamb dry and put it on a shallow platter. Season all over with salt and pepper.
3) Pour half of the vinaigrette over the lamb, turning the meat to get it well coated. Marinate in the refrigerator for two hours. Remove the marinated meat from the refrigerator about 30 minutes before cooking.
4) Preheat the griddle (or large cast iron frying pan) and oil it lightly. Remove the lamb from the marinade and griddle for about 15 to 20 minutes per side for medium-rare. (Adjust the time accordingly for other degrees of doneness.) Transfer to a cutting board and let rest for 10 minutes.
5) Toss the mache (or mixed baby greens), mint leaves, and spring onions in a bowl and dress the salad with about three tablespoons of the reserved vinaigrette. Season with salt and pepper.
6) Slice the lamb into thick slices and serve with greens.