I'm about to put up a big pot of Bolognese sauce. Two similar recipes, both from trusted sources, both calling for very similar ingredients, give slightly different simmering instructions. One says, "Bring the sauce to a boil, then lower the heat and cover. Slowly simmer for 1 1/2 to 2 hours, stirring now and then until the sauce is very thick." The other says, "...bring to a simmer. Reduce the heat to low so that the sauce continues to simmer just barely ... until the liquid has evaporated, about 3 hours." No mention of a cover in this second recpe (and there wouldn't be if the liquid needs to evaporate).
So -- cover or no cover? And how long should a Bolognese sauce simmer?