This recipe calls for 2 TB tomato paste in the marinade. I assume it is to thicken it so more stays on the meat and veggies. I don't want to open an entire can of tomato paste just to use 2 TB. Can I substitute anything else in its place?
Use your can opener on both ends of the can. They will both continue to stick to the tomato paste, especially the one on the bottom because gravity's had its way. So slowly pull up and grab one edge of the top lid. Then slide it off, pressing it against the lip of the can. Everything that was attached to it will now still be in the can. Carefully discard it.
Now, use a couple fingers (or your thumb) to press firmly on the bottom lid, being sure not to let it rock from side to side. It will slowly rise in the can, pushing a clean cylinder of tomato paste out the top of your can. (Just don't squeeze the can too hard.) When you've extruded the desired amount, use a flat blade or a credit card to scrape it off and add it to your ingredients.
Place the open can in a baggie (or vacuum seal) and then into a cool part of the fridge (toward the back). (If using a Ziploc style bag, seal it all the way, then barely open it, squeeze out as much air as you can and close the seal back up without letting any of that air back in.) When you need more, just pick up where you left off. It'll last a long time, even longer if you dip it in olive oil first. If you ever see it starting to darken a bit, pull it out, extrude that part and scrape it off before returning it to the fridge. Along the way however, be sure to never touch the actual product with your hands or fingers, as you'll impart bacteria certain to reduce its shelf life.