Put enough water in the stock pot so that at least half the filter can be submerged.
Bring the water to a boil and slowly add the baking soda. (The soda will fizz up, so add it about a tablespoon at a time.)
Put the range hood filter in the boiling water. (You may need to clean one half at a time, depending on the size.)
Boil for a few minutes, remove carefully, and rinse under hot tap water until the water runs clear.
If the filter is still greasy, refill the pot and start over.
Once the filter is clean, prop it up and allow it to air dry before replacing it in the hood vent.