I have never walked to this monastery, though I had hoped to do it back in 2012. The alternative turns a 4.5 km straight shot walk alongside the road into a 10 km trianguluar trip southwest from Mansilla to the monastery and then back north to the official camino. It is a Cistercian monastery, and
Gronze raves about it. I can’t find any wikiloc tracks, but it seems to be marked from Mansilla, and the discussion in that 2012 thread would make it pretty easy to find with something as basic as google maps. I woulldn’t take this route to avoid the road, but I would definitely take it to go visit the monastery!
I have a vague memory about an alternative route that avoided León altogether, swinging around it on the south, but I can’t find anything about it using the search function. I think it had to do with avoiding the city of León during covid. But that’s not what the OP is talking about.