Recipe Notes

Texture: Shallots tend to lose their flavour when cooked, yet incorporate more fully into sauces or custards whereas onions, even if chopped finely, will largely maintain shape and texture so there will be bits of onion in the custard or sauce.
Flavour: The flavor of an onion, even a mellow white onion, is a bit more agressive than a shallot and much bolder when cooked than a shallot. They are sweeter and more mild than either yellow or red onions and have a pleasant crispness when used raw in salad dressings and grain salads. When cooked, the flavour of a shallot is much more subtle. Shallots and onions are therefore not swappable in a recipe. There will be a significance, and possibly unintended difference in the resulting flavour and texture of the recipe particularly if it's a sauce like this, with a sense of subtlety.
I confess that I was in a pinch one time I made this sauce and used the white ends of green onions as I felt they would be the closest solution to a pleasing substitution. They worked well. That said, I always like to have a shallot or few in my fridge. They last a long time.
Interestingly, I left the lemon out one time as Vanja detests lemon in savory foods, and the sauce was also absolutely delicious!