Cut pumpkin in half and scoop out pulp and seeds. Wash and dry seeds. Place pumpkin halves cut-side down in a baking pan. Mix seeds with butter and salt and black pepper; spread in a single layer on a baking tray.
Bake pumpkin halves on the top rack of the preheated oven until liquid starts to collect in the pan, about 1 hour. Bake pumpkin seeds on the bottom rack until toasted, about 45 minutes.
Meanwhile, place potatoes into a large saucepan and cover with water; bring to a boil. Reduce heat to medium-low and simmer until soft, about 10 minutes. Drain and let cool.
Cool pumpkin halves for 10 minutes. Peel off and discard skin. Let cool completely, about 15 minutes more.
Combine potatoes, pumpkin, tomatoes, parsley, Himalayan salt, thyme and white pepper in a large bowl; mash together.
Puree potato mixture in a blender in small batches, pouring each batch into a slow cooker and letting blender cool between batches.
Stir coconut milk and milk into the slow cooker. Cook on Low until flavors combine, about 4 hours. Serve toasted pumpkin sides on the side.