Skipping breakfast and lunch, and just eating dinner. One meal a day.
One meal can be enough food to get your nutrient needs (protein, minerals, non fat-soluble vitamins, fibre to keep things moving) if you eat nutrient-dense foods like liver, vegetables, etc.
And for me at least, it is far easier to do this than any other diet plan. I don't "wake up" my appetite until dinner, and while it sleeps it doesn't nag me to eat... whereas if I just eat small meals, I'm constantly being nagged to eat more with hunger feelings. Especially after you get used to this pattern, your body doesn't expect to eat until dinner... and you don't tend to get hungry unless you are coming up on a normal mealtime.