Place a skillet over medium-high heat and add the oil. When hot, add the onions and cook until very soft, about 3 minutes. Add the cardamom and fennel seeds and cook for 30 seconds. Set aside. Put the pears into a food processor along with the mint and onion mixture. Season with some salt, and then puree until it has a smooth but slightly chunky texture. Store in an airtight container in the refrigerator.