I split 1 lb of bacon in half so that I have 32 slices, it's easier to work in the pan and use on sandwiches that way.
Gently pan fry the bacon on medium/med-low. You want it red-brown, shiny, and chewy, not dark brown, dried out, and crispy.
Remove 2/3 to 3/4 of the bacon grease, but leave some in the pan.
Raise the temp to 6-8 of 10, where the grease starts to smoke a bit.
Remove a pounded, brined chicken breast from the brine. Use paper towels to pat it dry, you don't want it dripping anywhere.
Fry the breast in the bacon grease with a lid on the pan for 2-3 minutes, then flip, and fry another 2-3 minutes.
Remove the breast from the pan and put it on a paper towel to rest and drain.
Set the chicken aside and let it rest until it is no longer hot to the touch, and does not leak juices when cut. If you try to complete the paninis while the chicken is still hot, it'll release too much moisture, ruin the bread, and keep the bread from toasting.
Some people prefer to slice the chicken into strips here so that the cheese runs between the strips when you press it on the panini. Sometimes I do, sometimes I don't.
Heat the panini grill to medium-high.
Slice the tomato and onion, if not already sliced.
Spread ranch on the bread slices.
Layer the sandwich from bottom up with 4 (half) slices of bacon, onion, chicken, cheese, tomatoes. This order, in my experience, maximizes juiciness while minimizing runoff and mess.
Stop here, salt & pepper your tomatoes.
Add the final 4 (half) slices of bacon on top of the tomatoes, then add the top piece of bread that's been spread with ranch.
Use a steel spatula to place the sandwich on the hot panini grill.
Carefully set the lid down and press the grill on the sandwich. Watch to be sure the top layers don't slide around and the whole thing stays together.
Grill for 3-5 mins until the bread gets toasted and the cheese is melting out the sides.
Remove from the grill, slice in half, and serve. It may need to rest a few minutes to cool down and consolidate before eating so that it doesn't burn your mouth or fall apart.