In a saucepan, combine the potatoes and crushed garlic with enough cold water to cover by 1 inch; season with salt. Bring to a boil over medium-high heat and cook until the potatoes are just tender, 20 to 25 minutes. Drain; set aside.
Meanwhile, in a large bowl, whisk together 2 tablespoons olive oil, the mustard, lemon peel, lemon juice and finely chopped garlic; season with salt and pepper.
In a large cast-iron skillet, heat 3 tablespoons olive oil over medium heat; add the sliced garlic and cook until golden, 1 to 2 minutes. Using a slotted spoon, transfer the garlic to paper towels to drain. Add the bread cubes to the pan, season with salt and pepper and toast, tossing occasionally, until golden, 5 to 7 minutes. Transfer to a plate.
Wipe out the skillet. Add the remaining 1 tablespoon olive oil and heat over medium-high heat until hot. Season the steak with salt and pepper, add to the pan and cook, turning once, for 6 to 8 minutes for medium-rare. Transfer to a cutting board and let rest for 5 minutes before thinly slicing against the grain.
Quarter the potatoes and add to the vinaigrette, tossing well. Add the onion and spinach, season with salt and pepper and toss again. Divide the salad among plates and top with the steak, garlic slices and croutons.