Preheat your oven to 350°F (177°C). Carefully cut chestnuts in half with a large kitchen knife. Place halved chestnuts on a cookie sheet, nut-side-up and shell-side-down. Bake for 30-35 minutes. Remove from oven and let cool.
Once cooled, remove nuts from the shells and inner testa (the thin papery layer coating the nut).
While the chestnuts are baking, rinse and de-seed your crabapples. Set aside.
Put chestnuts, dairy, and salt into saucepan on your stovetop. Try to use a pan size that allows the chestnuts to sit in a single layer covering the entire bottom of the pan and have the dairy just covering the chestnuts.
For first 5 minutes, turn heat to medium, keeping LID ON. Then turn the temperature to the lowest setting on your stove (keep LID ON), and cook for ~20 minutes. You may need to remove the pan from the heat from time to time if the dairy starts to bubble and get too hot. Try not to let it boil or the milk may break/separate.
Add crabapples and rosemary to pot and REMOVE LID. Continue to cook for another ~35 minutes over low heat to reduce water content and concentrate flavors. Stir occasionally.
Remove rosemary, then remove pan from heat. Allow contents to cool down until they aren’t scalding hot. Transfer to food processor and blend until silky smooth. Scrape the sides and re-blend as necessary. Taste and modify as necessary - more salt, more milk, etc.
Chestnut crabapple mash is best served warm. It can store in the fridge in an air-tight container for up to 1 week.
Chestnut crabapple mash https://www.tyrantfarms.com/chestnut-crabapple-mash/