The crust: add potatoes to the slow cooker and cover with cold water.
Cover and cook on LOW for about 4 hours, until the potatoes are tender when pierced with a fork.
Drain off water and increase heat to high; cook for about 30 minutes, until dry.
Add the sour cream, garlic, salt, and pepper; using a potato masher, mash the potatoes to your desired consistency; transfer potatoes out of slow cooker and refrigerate (clean slow cooker for use with the filling).
The filling: place a large saucepan over high heat and add 3 tablespoons butter.
Add in the flour and cook, while stirring constantly, for about 5 minutes, until lightly browned.
Add in the stock; continue cooking/stirring for 15 minutes, until the mixture thickens enough to coat the back of a spoon.
Add in nutmeg and salt/pepper.
Place a saute pan over med-high heat; add in remaining 1 tablespoon butter.
Add in leeks, carrots, and celery; saute for about 10 minutes, just until soft.
Transfer to the slow cooker and add the chicken and sauce; gently fold in the peas and fresh tarragon.
Cover and cook on LOW for about 4 hours, until the vegetables are tender and the chicken is cooked through.
Spoon the potato crust over the top of the filling; cook for about 1 hour, until the potatoes brown at the edges.