“How do I get the bechamel (top layer on my moussaka) to be a more firm layer rather than a slightly soggy creamy yummy mess?“
I do love a good béchamel. That was the first sauce that I properly learned to wing without a recipe, though technically it was a mournay sauce because I added some cheese. But the béchamel was the base, and it’s a tasty addition to many meals.
Your basic béchamel is milk thickened by flour. Most people in the US think of gravy when they think of a sauce thickened with flour, and their next thoughts go towards lumps. Accursed lumps: the scourge of flour thickened sauces everywhere.
Lumps happen because the flour, when it comes in contact with a hot liquid, will gelatinize instantly, which is why you’re using it. However, if the flour is even vaguely tightly packed when it hits the liquid, the outside layer will gelatinize forming a liquid-tight barrier around the rest of the flour. So people find ways of keeping the flour from forming these lumps. You can be very careful when adding the flour; you can use a flour that’s designed not to clump, or you can surround the flour with some sort of fat, which will keep the water away from it for a time.
That last suggestion, the surrounding with fat, is the most widely recognized way in the food press, and the most popular method of surrounding the flour with fat is the roux. A roux is when you heat up a fat (often butter, but anything fatty works), add in the flour, and cook the flour until it browns a little. Or until it browns a lot, depending on the recipe. For a white sauce, though, you don’t want a dark flour. The reason the roux is so popular is because it cooks the flour a bit, which removes its unpleasant raw flavor.
So: milk, butter, and flour are the traditional components of a béchamel. The milk is the base, the butter is for clump protection, and the flour is to thicken. Flour thickens by absorbing hot water and combining that water with its starches. The starches, which are tightly wound in their dry state, spread out in every direction and taking up a lot more space than they did before.
I have a box filled with extension cords, power cables, speaker wire, and so on. When freshly purchased, these cables are nicely wound and tied of with zip ties or similar. They stack neatly and hardly take any space. After I use them and put them back into the box, I rarely put the same care into winding them up as the manufacturer did. Pretty soon, the cables are all wrapped around each other, filling easily three times the space that they used to. Attempting to remove one cable brings out all the cable. Flour is just like that, and that’s how it thickens the milk (or whatever other liquid you happen to be cooking with).
So, because flour is the structure for your béchamel, the best way to add more structure is to add more flour. The sauce will thicken some when it cools as well, so don’t think that your thickest consistency will be when the sauce is at near-boiling.