Escallion = scallion = green onion.
I think it's possible to substitute regular onions in some situations, but there are a lot of variables.
First off, scallions have a sharper, more grassy flavor than onions, though they're not as strong.
Then there's the question of which kind of onion you want to substitute. White onions have some of that same sharp flavor, and are quite strong. Yellow onions are much milder and sweeter, and might not be such a close flavor match. Red onions are somewhere in between, in my experience. I would probably try white onion, but in much smaller quantity than called for of scallions.
It's also important to consider how the scallions are being used. If they're being cooked into the recipe, that's very different than if they're being added right at the end, or even as a garnish. I think the substitution is less of an issue if the scallions are to be cooked. If they're to be more or less raw, I'd suggest white onion, not too much of it, and sliced very very thinly, as a raw white onion is really strong.
Whatever you try, bear in mind that it won't be exactly the same. But that certainly doesn't mean it'll be bad--just different.
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.
Best Answer
As a child, I used to hate the feel and taste when I conciously bit on a piece of onion (strongest offender: medium-sized chunks, sautéed glassy but still firm). Sometimes that still happens.
However, I use onion a lot myself now in cooking. Lots of dishes really need the kind of taste onions can supply; even though that taste may by itself be a bit gross, it's really useful in the mix. (This is true for many other ingredients as well!) I can only speak for myself, but very likely, there are some dishes where you don't even notice that onions are in, but they are – and you wouldn't like it if they weren't!
Do use onions, just do it consciously. There are broadly three ways of preparing them that I consider effective, each suited to different cases:
Ideal for light/creamy sauces or soups.
Perhaps you won't be able to befriend yourself with all of these preparation methods, but you should give them a try.