Season the chicken with salt and pepper. Heat a large pot with a lid or a Dutch oven over medium-high heat with olive oil, two turns of the pan. Brown the chicken on each side, then remove to a plate. Add the ham to the pan and brown the cubes, 2-3 minutes; remove to a plate.
Add the butter to the pot and melt. Add theleeks and garlic and season with salt and pepper. Cook to soften, 3-5 minutes. Deglaze the pan with the wine, scraping up the bits at bottom of thepan. Whisk up the stock, Worcestershire and mustard and pour into the pan. Add the potatoes, carrots, chicken and ham and cover.
Cook in a 325°F oven or over a medium-low flame for 40-45 minutes, stirring occasionally.
To serve, halve the lemon and squeeze the juice around the pan; garnish with lots of tarragon.