Can I make a bechamel sauce for lasagna a few hours ahead and refrigerate it?
You can. Just use a tad more liquid than the recipe you use calls for, as it will thicken considerably upon refrigeration.
put some saran right on the surface so it doesn't develop a skin...and for just a few hours I wouldn't bother with the fridge. Room temp would be fine.
I'm always taken a back when I see dangerous and frankly irresponsible advice on food safety posted here. Bechamel will be a virtual petri dish once it hits the danger zone. To make it in advance, cook it, then cool it down quickly over an ice bath. Cover it with plastic wrap directly on the surface to prevent a skin forming, and refrigerate immediately.
Do not under any circumstances leave this out for any extended time.
Out of curiosity, since creme fraiche can (or should) sit out on the countertop overnight, what is the difference with bechamel?
Pegeen, crème fraîche and béchamel are very different. In the case of the former, cream is being encouraged to sour, and a warmer temperature than the refrigerator is clearly required for that to happen. The souring process is simultaneously a preserving one. As for béchamel, a lot of milk has been cooked up to a high temperature. As with any food, it needs to be cooled to 70 degrees or below within 2 hours, then refrigerated and brought to 40 degrees or below within another 4 hours. Cooling/refrigeration is the only thing standing between the sauce and spoilage. Time and temperature abuses are among the most common elements of food-borne illness.
That said, Beth, you can certainly prepare your béchamel before hand. Just be sure to adequate cool it. Pour it out into a shallow pan or even baking dish. Lay a sheet of plastic wrap directly in contact with its surface, as Pat suggests, and use a sharp paring knife to poke several holes in it, then refrigerate it.