For the lamb: Put the onion into 1 large shallow dish in which the noisettes will fit into one layer, or divide the onion into 2 medium-to-large freezer bags. Add the oil, lemon zest, and cumin (dividing equally if you are using bags). Give a good stir and then add the lamb. Cover the dish or tie up the bags and leave, turning or squishing respectively at half time, as long as you've got; frankly, though, it should be for at least 10 minutes and preferably not in the refrigerator. Preheat the oven to 425 degrees F. Put a non-stick or cast-iron pan on the stove. Remove the noisettes from the marinade; you don't need to wipe then dry, just brush off the bits of onion. Sear each side for 1 to 2 minutes, then transfer to a baking pan and then to the preheated oven for 10 minutes. The meat should be right for pink, but not bloody, you may need a bit longer if the meat started off very cold. You will need to check for yourself and when cooked as you want, remove to a warmed plate.
For the sauce, put the tahini in a bowl and add the garlic and salt. Stir with a wooden spoon, adding the lemon juice as you do; it will seize up here, but don't worry because it will loosen later. Slowly add some water ( I find I can use about 1/2 cup), pouring from the measuring cup so that only a little goes in at a time, and keep stirring. When you have a smooth mixture the consistency of heavy cream, stop adding water. Put into a bowl with a spoon and sprinkle with the additional ground cumin.
For the spinach: Thaw it in the microwave according to the package's directions and then cook briefly to warm, about 1 minute more. Remove from the microwave and place onto a clean plate. Drizzle with the olive oil and then top with the toasted sesame seeds.
Serve the lamb noisettes with the spinach and the garlicky tahini on the side.
Recipe courtesy of Nigella Lawson