Turn crockpot on high and place chicken breasts, quartered onion, and herbs in it.
In a medium saucepan, melt 3 tablespoons of the butter and stir in flour.
Cook for a few minutes, until flour just starts to turn golden.
Add the chicken stock and heavy cream, whisking constantly.
Let simmer for 3-5 more minutes until thickened.
Add salt and pepper.
Pour over everything in the crockpot and cook on high for 3 1/2 or 4 hours, until the chicken is shreddable. You can also cook on low for 8 hours, but I find this dries out the chicken.
Halfway through cooking, check the liquid in the pot, add more chicken stock if it starts looking dry or too thick. The consistency should be slightly thicker than cream.
When chicken is done, remove herb sprigs (if you used fresh herbs) and onion quarters.
Shred chicken.
Stir in sour cream, poppy seeds, parsley and chives and leave covered in crockpot.
Turn oven on to broil on high.
Crush package of Ritz and sprinkle over top of the mixture in the crockpot.
Drizzle remaining 4 tablespoons melted butter over the top.
Remove ceramic insert from crockpot and place in oven.
Broil on high for just a few minutes (keep your eye on it!!) until the crackers are slightly crisp and darker in color.