While the oven preheats, remove the ham from the packaging and allow it to come to room temperature.
Line a large cast-iron Dutch Oven with heavy duty aluminum foil - you’ll thank me later.
Place the ham into the foil lined pan, cover with parchment paper and then with more foil. If it’s easier and depending on the shape of your ham, you may be able to make a parcel out of a layer of parchment and a layer of foil. Wrap the ham first in the parchment and then in the foil.
Bake the ham, covered or wrapped in parcel, for 40-45 minutes to warm through.
While heating the ham, combine the honey with the mustard in a small saucepan. Then pour the hot water into the honey jar, replace the lid, and shake well to dilute any remaining honey. Add the hot honey-water to the saucepan with the honey and mustard and gently warm the glaze in the saucepan. When the glaze is warm, whisk in the butter a tablespoon at a time. Set glaze to the side and allow to come to room temperature.
When the ham is warmed through (a digital thermometer inserted into the thickest part of the ham should read 125°-135° at this point), remove it from the parcel or uncover it and turn the oven temperature up to 425°. Keep the heavy-duty aluminum foil on the roasting pan - it’s about to get messy.
While the oven temperature is increasing, use a sharp knife to score the ham in the classic diamond pattern or against the grain and in even intervals across the top of the ham. This will allow the glaze to set into the ham and make the final step of carving a bit easier should you master the shellac-like surface texture of a perfectly glazed holiday ham.
When the oven has reached 425°, mix in the brown surge and flake salt to the now room-temperature glaze (you don't want to add the sugar and salt to hot glaze or it will dissolve and you won't get as much crusty bits on the surface of your ham). Now, brush approximately a quarter of the glaze onto the surface of the ham. Roast the glazed ham uncovered for 15 minutes.
After 15 minutes remove the ham and apply more glaze. Repeat this glazing about every 15 minutes until the glaze is gone or until the ham has achieved a nice dark-golden crust.