Preparation Slice steak against the grain into as-thin-as-you-can-get-them strips (this is much easier to do when the steak is cold). Transfer to a large bowl and let sit at room temperature while you make the slaw. If you have time, slice the steak before you start cooking; an hour is ideal to completely take off the chill. Toss sliced fennel, red onion, salsa verde, 1/2 tsp. salt, and 1 Tbsp. fennel fronds in a large bowl to combine. Season slaw with a few grinds of pepper and toss again. Spread cut sides of focaccia with 2 Tbsp. mayonnaise each; transfer to plates. Divide steak between bread and crown each sandwich with a big handful of slaw. (You may have a small amount left—nibble on it or serve on the side.) Top sandwiches with more fennel fronds and sprinkle with salt.
Preparation Slice steak against the grain into as-thin-as-you-can-get-them strips (this is much easier to do when the steak is cold). Transfer to a large bowl and let sit at room temperature while you make the slaw. If you have time, slice the steak before you start cooking; an hour is ideal to completely take off the chill. Toss sliced fennel, red onion, salsa verde, 1/2 tsp. salt, and 1 Tbsp. fennel fronds in a large bowl to combine. Season slaw with a few grinds of pepper and toss again. Spread cut sides of focaccia with 2 Tbsp. mayonnaise each; transfer to plates. Divide steak between bread and crown each sandwich with a big handful of slaw. (You may have a small amount left—nibble on it or serve on the side.) Top sandwiches with more fennel fronds and sprinkle with salt.