Temperatures for the fillet, measured in the middle: pull at about 48C for rare, 52C for medium rare, 58C for medium, and rest five minutes. The centre keeps rising three to five degrees off the heat. Past about 60C the meat dumps its juice and turns chewy, which is a waste of this cut.
The yoghurt is the step to be careful with. Take the pan off a proper simmer first, stir it in slowly and only warm it through. Yoghurt is acidic and low in fat, so heat, salt and hard stirring will split it where the double cream is unbothered. Creme fraiche in its place is much steadier under heat.
Do not wash the pan between the beef and the bread. The fat and fond the steaks leave behind is what makes the sourdough worth eating.
Everything lands at once, so chop and measure before anything goes on the heat.