I spent a lot of time on the underpainting to get the proportions right. The bag leans so far to the right, and we often want to "straighten" things out. Drawing is basically just making your brain stop editing what you see.
I quickly put in the basic colors of the onion first, with saturated, clean colors. I knew I'd spend a lot of time on the complicated bag, and I wanted the onion to be fully incorporated even if I neglected it while the bag developed.
I blocked in the basic structure of the bag with flat brushes. I just bought some new long-bristled Princeton brushes I like, in both flats and filberts. I use a large sable filbert for the background after I have blocked in the color with a bristle brush.
I used smaller filbert bristle brushes, and in some cases a small flat bristle, to refine the various planes of the crumpled bag. At this point I am very happy I spent so much time on the underpainting - the basic shapes are right so I can focus on color rather than drawing.
I experimented with Prussian blue instead of my usual Ultramarine, it was a nice change for me. The whole palette was:
Cad Red Medium
Cad Yellow Deep (an orange I love)
Cad Yellow Medium
Raw Umber (just for the underpainting)
Mars Black (yes, I just started using black last week for the first time ever)
I used to get really frustrated where a light area meets a dark area - because so much white mixed with so much dark (blue, umber, black, or all three) makes a really chalky, ugly midtone. So I have started pouring a ton of color into my midtones and it has really helped.