Combine the broth and the bouillon cubes in a microwave-safe container (I like to use a 2-cup pyrex measuring cup). Use all chicken for chicken gravy or all beef for beef gravy. Microwave for 1 minute and stir. Repeat until the bouillon cubes have dissolved (about 2 minutes). Do this in a small saucepan on the stove if you don't use a microwave.
Melt the butter in a medium saucepan over medium-high heat. Add the flour, onion powder, rosemary, sage and thyme. Whisk to combine and cook for 1 minute, whisking occasionally to cook out the floury taste. Whisk the hot broth into the flour mixture. Bring the gravy to a boil, decrease the heat to a simmer and whisk until it is as smooth as silk, about 1 minute. Taste and season with salt and pepper, as desired.