If this feels intimidaring and you get stuck on the first line, just write something, whatever, to get you started. You can always go back and change the first sentence later.
Step 1. start writing the book Step 2. write the rest of the damn book Okay that's far too glib but here is my sincere advice: 1. have an idea of who your characters were, are, and want to be 2. nail down a plan for at least the opening third and have an idea of where you want to go beyond that. It might come to pass that scenes you come up with during that first third change the trajectory of your characters, so plotting it out in detail is pointless. 3. you don't have to write things in order. If you want to write the ironic ending first, go for it. Then you can foreshadow. If you get stuck in a certain scene, skip ahead, or back, and write something that progresses the story, even if it's not in the chronological order of events.
While I appreciate the star studded lineup. Some are not there for their literary genius. And I quote... "He was perhaps forty years old. He had thick black hair, shiny, beautifully cut, and the kind of mid-brown skin and regular features that could have made him Indian, or Pakistani, or Iranian, or Syrian, or Lebanese, or Algerian, or even Israeli or Italian. His passport was British..." Could his nationality not be condensed to ...his nationalilty well masked, somewhere between middle eastern to southeast asian? 400 pages later we get... "He was medium height, medium weight, expensively dressed but a little rumpled. He was maybe forty years old. He had thick black hair, shiny, beautifully cut, and the kind of mid-brown skin and regular features that could have made him Lebanese, or Algerian, or even Israeli or Italian." This does NOT pass for literature - fiction, thriller or otherwise. 100 million copies sold by this author... The mind boggles.
As you should well know, popularity is not a mark of quality. There is a lot of dreck on the market, but people buy it. Not to mention padding the word count.