Meant by “cook until the oil separates” in Indian curry recipes

I own the same book and was similarly surprised when I read that instruction, but in the section on ingredients, the author does mention a particular variety of onions called pink onions. The mention is on pg. 32, and there is a picture of a pink onion on the upper left of pg. 34.

Here's an excerpt from pg. 32:

"The longer the onions are fried, the browner they will get and the deeper the color of the curry will be... When the onions are fried until only light pink in color, they will impart a sweetish taste to the curry. Certain varieties of onion, like Spanish onions, are too sweet to be appropriate for curry-making. The most appropriate from the taste point of view are the French and the small pink English."

From 50 great curries of india

I've never seen pink onions in any of the local markets (in northwestern US), I used white onions and sauteed them just to the point of browning. The recipe turned out fine, but it's a lot of onion. (The recipe calls for 2 large onions, finely chopped. It's a recipe that produces 2 servings.) Personally, I'd go a little lighter on the amount of onion or use yellow onions (despite the author's recommendation against Spanish onions), but that is just personal preference from a Western palate.

Note: it looks like the current printing of the book is different from the one I own, so the page numbers I cite may be off, but if you search inside the book (on Amazon) for "pink onion" you can see the page I've cited and the picture of what the author calls a pink onion.

These are the factors to consider when using tomatoes in Indian cooking:-

1) Does the recipe need pureed or chunky tomatoes to contribute bulk to the gravy? >> if puree then canned is fine or even tetrapak puree. But stick to fresh tomatoes if you need to increase the gravy's volume. E.g.: Paneer Butter Masala where tomato puree is better vs. Matar Paneer where diced tomatoes bring out more tangy spicy flavor.

2) Do you need to keep the peel on or lose it completely? If you need to lose the peel, peeled & canned tomatoes can save much time over having to boil or microwaving the tomatoes.

3) What is the cutting style required in the recipe? finely chopped, coarsely chopped? This point is relevant in case the recipe needs only semi cooked tomatoes.

4) Are the tomatoes being used to add bulk or for flavor? Canned tomatoes do have a slightly different flavor specially if a preservative brine solution has been added.

5) In salads or Indian dishes like bhel puri, the way tomato pieces are added also contribute to aesthetics and make food more appetizing. so canned tomatoes are a stric no-no for such dishes.

And overall- Use Fresh Tomatoes >> tomato is one of the veggies which can be chopped fast & painlessly. To save time, turn your tomato over when chopping (fleshy side up, peel side down)