Prep your workspace. First things first – make sure your slow cooker is large enough to accommodate your ham. If it seems too tight, you might need to trim a bit off the ham to make it fit properly. Nobody wants a ham lid that won't close!
Create your flavor base. Drain the pineapple slices, RESERVING THE JUICE – this liquid gold is a key part of your glaze! Set the pineapple slices aside for now.
Mix the glaze. In a bowl, combine the brown sugar, reserved pineapple juice, honey or maple syrup, Dijon mustard, cinnamon, cloves, and nutmeg. If you're going the boozy route, add your bourbon or rum now. Whisk until smooth and the sugar begins to dissolve.
Prep the ham. Place your spiral-cut ham in the slow cooker, cut side down. This orientation helps keep the ham moist as it cooks.
Apply the flavor. Pour about two-thirds of the glaze over the ham, using your hands or a brush to work it between the slices. Tuck some of the pineapple slices between the ham layers and arrange the rest around the sides and on top. Save those few remaining pineapple rings for garnish later!
Let the magic happen. Cover and cook on LOW for 4-6 hours. A good rule of thumb is about 30 minutes per pound of ham. Since the ham is already cooked, you're really just warming it through and infusing it with flavor.
Baste occasionally. If you're around, baste the ham with the cooking juices every hour or so. If you're out living your life, don't worry about it – the ham will still be delicious!
Finish with a flourish. About 30 minutes before serving, spoon the remaining glaze over the ham and increase the slow cooker to HIGH for the final stretch. This helps caramelize the exterior a bit more.
Reduce the sauce (optional but recommended). Transfer the cooking liquid to a small saucepan and simmer until reduced by about half and slightly thickened. This concentrated glaze is liquid gold!
Rest and serve. Carefully transfer the ham to a serving platter (it might be so tender it tries to fall apart – a good problem to have!). Let it rest for 10 minutes before slicing. Drizzle with the reduced sauce and garnish with the reserved pineapple slices.