Wild onion is actively growing in colder weather than fescue. I have had success with 2,4D in the early spring, before the fescue starts growing.
One spot application when the snow breaks (I aim for the second half of February), and another shortly before the first mow of the season (when the shoots are sticking up high above the grass in clumps).
I mix surfactant in with the 2,4D to get it to spread on the waxy coating, and also bruise the weeds with a rake just before spraying. Spray it on the un-cut plants. That give more surface area for absorption.
A 3-way might be better, but I've got trees in the affected lawn. Plain 2,4D was the safer route for me, and in spot treatments, it worked surprisingly well on wild garlic. It took 2 years (4 applications), but in that time, I've gone from more weed than grass, to almost no wild garlic left. Before that, the onion smell when mowing could get overpowering.