Spinach is traditional but I have made wonderful gomae with radish and turnip greens as well. It's great to make ahead.
This is a "cheat" or a hack because we're using tahini (because it's easier to get and you are more likely to have already) instead of pounding or grinding sesame seeds.
Adapted from: https://www.eatingwell.com/recipe/251348/japanese-style-spinach-gomae/
Bring a pot of salted water to a boil and have a bowl of ice/cold water nearby. Drop the spinach into the boiling water and after about 20 seconds, fish it out and submerge it in the ice/cold water to stop the cooking process.
When it's cool to touch (almost right away), squeeze out as much liquid from the spinach as you can.
In a small bowl, mix together the tahini, rice vinegar, soy sauce, water and mirin. You can mix this by hand or pop it in a magic bullet/blender and give it a quick whizz.
Mix the dressing/marinade with the spinach and let it sit to absorb the flavours for at least 15 minutes.
Right before you serve the gomae, give it another stir (the dressing has a way of going to the bottom of the bowl), drizzle with sesame oil and sprinkle with toasted sesame seeds. Serve chilled or, my preference, at room temperature.