Turn on the oven and heat it up to 374°F (190°C).
Rinse the plums and remove the stems. Make one lengthwise slit in each plum, going only halfway through, and take out the stone.
Tuck a walnut half into each opening. For smaller plums, a walnut quarter fits better.
Spread about ⅓ cup (115 g) of the honey over the bottom of a baking dish, just big enough to hold all the plums snugly.
Arrange the stuffed plums in the dish, nut side up, packing them close together so they hold their shape.
Drizzle the remaining ⅔ cup (235 g) honey over the top, making sure every plum and walnut gets coated.
Dust evenly with 2 tablespoons of cinnamon, or more if you like a stronger spice.
Roast for 15 minutes at 356°F (180°C). Then turn the heat down to 329°F (165°C) and roast for 50 more minutes without stirring.
Meanwhile, sterilize your jars in boiling water for 5 minutes, or put them in the oven for the final 10 minutes of roasting.
Carefully transfer the plums to the hot jars one by one, adding a spoonful of pan syrup after each layer. Keep them loosely packed.
Top off each jar with syrup so the plums are completely submerged, tap out any air bubbles, and seal tightly.
Flip the jars upside down, cover with a blanket, and let them cool slowly to room temperature. Check the lids, then store in a cool, dark place.