Place the onions and the garlic in the crockpot. Sprinkle the chicken pieces with salt and pepper, then slather them with the mustard and place in the crockpot. Pour the lemon juice and chicken bouillon on top and then finish with the tarragon sprigs (or sprinkle on dried tarragon).
Cook on low heat for at least four hours, giving it a stir, if possible, along the way.
Before serving this crockpot lemon chicken recipe, remove the tarragon and discard. Remove the chicken, which should be falling off the bone, and set aside. Stir the cream into the sauce and then return chicken to the crockpot to serve.
Note: For extra lemon flavor, place a couple of slices of lemon on top along with the tarragon before cooking.