The night before, combine the green tomatoes and onions in a large glass or ceramic bowl and sprinkle with approximate 3 tablespoons (45 mL) coarse sea salt. Toss slightly.Cover with plastic wrap and allow to sit overnight.
The next morning, drain off any liquid and thoroughly rinse and drain the onions and tomatoes.
Combine all the ingredients, with the exception of the pickling spice, in the Instant Pot and stir well.
Make a small well in the middle of the ingredients and place the cheesecloth-wrapped pickling spice in the well, as close to the bottom as possible.
Close and lock the lid, ensuring that the valve is in the Sealing position.
Select Pressure Cook (Manual on some Instant Pots) mode and set cooking time for 5 minutes.
Once cooking time is complete, allow a full Natural Pressure Release (this can take approx. 25-30 minutes).
Once the Float Valve has dropped (happens when the pressure is totally released), carefully open and remove the lid.
Find and remove the cheesecloth-wrapped pickling spice.Stir well.
Carefully remove the Instant Pot’s inner liner and place on a heattproof surface.
Package the chutney in sterilized glass canning jars. Refrigerate, freeze or can.