Process in a food processor: toasted baguette slices, rosemary, garlic, salt and pepper along with 2 Tbsp EVOO. Use on/off pulses until you have moist crumbs - big pieces don't adhere well, so you're going for a smaller and uniform crumb size.
Working on a sheet pan, spread mustard on tops of fish fillets and press breadcrumbs on to cover the entire surface. (can be prepped to this point, covered and refrigerated up to 6 hrs ahead of baking).
400 oven. Bake fish until opaque in center, about 20 minutes.
Great to serve alongside roasted potatoes and braised greens with a balsamic drizzle.