A great sear is mostly about heat and dryness — and tallow is built for both. Its high smoke point means you can get a pan properly hot without it breaking down.
Start with a dry steak at room temperature. Moisture on the surface steams instead of browns, and that's the enemy of a crust.
Get your cast-iron pan screaming hot, add a spoon of tallow, and lay the steak down away from you. Then leave it alone — resist the urge to move it while the crust forms.
Flip once, baste with the foaming fat, and rest before slicing. That's the whole trick.
The same principles work for chops, burgers and even a good grilled cheese. Heat, dryness, patience.