Combine the flour and oil in a heat-proof (i.e., Pyrex) measuring cup. Stir it to blend it as completely as you can.
Microwave the mixture on high for 2:30 (two minutes and thirty seconds). Remove it from the microwave and stir it very well with a teaspoon or a fork, breaking up any clumps of flour. You may be able to get a smooth mixture (or maybe not), but that is the ideal.
Microwave again for 1:30. Remove and stir very well, until smooth. Be careful! This stuff will now be very hot, and the cup and the business end of the stirring implement will be hot enough to burn you severely. (The handle of the cup is usually safe to grasp--but don't take that on faith!)
Microwave for 45 seconds. The roux now will be hot enough to continue cooking and darkening while you're stirring it, so keep stirring it, scraping the bottom of the cup and breaking all clumps immediately, until you have a smooth paste. Then stir for another 30 seconds at least.
The roux should be dark enough for most purposes by now, but if you want it darker still, microwave it in bursts of 15 seconds or less, with long stirring in between. When it reaches the color you want, keep stirring until it has cooled enough to stop cooking. Or stir in the chopped onions, celery, etc. from the recipe you're working on.