Press Instant Pot Saute function button to heat up Instant Pot inner pot. When display reads not, add olive oil. Then add celery, carrots, and onion. Saute for 3-4 minutes until slightly soft. Add garlic and saute one more minute.
In a large bowl add cooked vegetable mixture to ground beef and sausage. Add 1/2 cup of the BBQ sauce and all remaining ingredients and mix well. This works best using your hands. Mix, but do not overmix or the meat will be tough.
Place a large piece of foil on your countertop, approximately double the size of your Instant Pot’s inner pot. Pour meat mixture onto foil and form into a round loaf that will fit into your inner pot with a little room to spare on the sides.
Place 1 cup water into Instant Pot inner pot, then place trivet in pot. Carefully lift meatloaf using the sides of the foil and place into pot. Drizzle 1/4 cup of the BBQ sauce on top of loaf. Tuck foil into the pot so it does not interfere with the lid placement.
Secure lid, ensuring the valve is turned toward sealing. Select pressure cook (or manual depending on your model) and set the timer to 40 minutes. When timer goes off, let sit for another 10 minutes (this is a “natural release.”) Turn the valve to venting to release any remaining pressure. Carefully remove meatloaf using the sides of the foil. Let rest 10 minutes then drizzle remaining BBQ on top. Serve in thick slices topped with additional sliced green onions.