The roast was super-easy. Just rubbed it with olive oil and spices, then put it into a pan in a preheated 450F oven until my probe thermometer said 135F. Pulled it and let it rest a few minutes until it reached a safe 140F before slicing. Very good.
The gravy was also very easy:
1) Slice onions and mushrooms (however many you want) and take out a saute pan that will hold them all -- but it doesn't have to be a saute pan; you can use a big stockpot, even, if you have to
2) Put some butter (anywhere from a tablespoon to several) into the saute pan and heat the pan on medium until the butter melts
3) Add the onions and mushrooms to the butter, and sprinkle a little salt and pepper over the top. If you have it, now's the time to add a teaspoon of garlic granules or a chopped garlic clove.
4) Let them cook on medium, stirring occasionally, until most of the liquid cooks away, then add just enough chicken stock or water or heavy cream to barely cover, and let it come back to the simmer. If you used chicken stock or water, you'll have to do step 5. If heavy cream, just continue to heat until it thickens a bit and you're done.
5) In a little bowl, mix a tablespoon or two of cornstarch with a tablespoon or two of cold water. Add to the simmering gravy and stir constantly until it returns to the boil. As thick as it is now is as thick as it will be, so if you need to do it again (to make it thicker) or add water/stock (to make it thinner), do that now.