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.