How to make roux-based sauce thicker

If your sauce is too thin, the problem is that your initial roux was either too thin (not enough flour) or you added too much liquid for the amount of roux that you made. Standard ratios are 1 Tbsp butter - 1 Tbsp flour - 1 cup liquid for a thin sauce, 2-2-1 for something in the middle and 3-3-1 for a thick sauce.

Once you've made the sauce and it's too thin, the best fix is to add a beurre manié, which is essentially butter and flour that you mash together in a small bowl and then whisk into your sauce. It's kind of like adding more roux after the fact. However, because it's not pre-cooked like the roux, you need to make sure you give it enough cooking time after adding to get rid of the floury taste.

As an alternative to corn flour, Arrowroot is a plant based starch of similar price to corn starch but with some better qualities

Important is sauces is that it does not cloud the sauce, and keeps a shiny appearance

Also, it doesn't require the heat level to set that corn starch does. So it can be added after a sauce is finished to increase thickness

Always mix Arrowroot with a little cold liquid (not milk) and then add that to the sauce

One excellent way to thicker almost any type of sauce is to use corn flour. It is inexpensive and easy to use. When you've decided you want to thicken your sauce separately mix some cornflower with a little milk or water to make a smooth paste and then add to you sauce making suer you stir. This works well for soups too.

A quick solution is to make a slurry. Add 1 tbsp flour to 1/4 cup liquid and stir til combined, if too thick add a little more cold liquid. BTW, if you add hot liquid it will expand too quickly. Once it is thinned out enough, add to the pot and whisk together. When it is thick enough, add equal extra amount of butter or oil to balance flavour.

If you have this problem frequently, one solution is to make more roux than you think you'll need, then reserve some after you've cooked it, but before you've added any (or much) liquid;

You can then add the roux in at the end ... the easiest way is to get some onto a wisk and then wisk it in, and keep repeating until you think you've gotten enough in.

It will thicken up with time if you leave it at a simmer, but I think that might be partially from evaporation, so you'll end up with less. It'll also thicken up more as it cools down, so you want it a little thinner than the thickness you want to serve it at, or it might turn into a giant lump.

The best way that I've found to keep the roux from being too thin is to slowly add the liquid, letting the roux thicken as it cooks. When it starts to get thick and I need more volume, I'll add more liquid. Repeat until the required volume is reached. Depending on what I'm making it for, I might add more butter and/or flour.