How to Make It
In a blender, process the lemon juice, lemon zest, brown sugar, 1/2 teaspoon pepper and the olive oil until smooth. Put the beef in a shallow bowl and add all but 2 tablespoons of the marinade; turn the steaks to coat. Cover and refrigerate for 1 hour.
Meanwhile, toast the paprika in a small dry skillet over moderate heat, shaking the pan, until the paprika is brick colored, 3 to 4 minutes. Transfer to a plate to cool.
In a small saucepan, boil the beef stock until reduced to 2/3 cup. Add the paprika, sour cream and yellow onion and cook over moderate heat until slightly thickened, 6 to 7 minutes.
Preheat the broiler. Brush the red onion slices with the reserved 2 tablespoons of marinade and arrange them on a broiling pan along with the red and yellow bell peppers. Broil the onion slices and peppers for about 15 minutes, turning occasionally, until charred all over. Transfer the peppers to a bowl, cover and let steam for 15 minutes. Coarsely chop the red onion. Peel, core and seed the peppers and cut them into 1/2-inch pieces.
Bring a medium saucepan of salted water to a boil. Add the asparagus and cook until crisp-tender, 3 to 4 minutes. Drain and cool under running water.
Preheat the oven to 450°. Heat the canola oil in an ovenproof skillet until shimmering. Drain the steaks and season with salt and pepper. Cook over moderate heat until browned, about 7 minutes per side. Transfer the skillet to the oven and cook the steaks for 4 to 5 minutes for medium rare. Transfer the steaks to a cutting board and let rest for 10 minutes.
Rewarm the sauce. Melt the butter in a large nonstick skillet. Add the Herbed Spaetzle and cook over high heat, stirring occasionally, until crisp and golden, 8 to 10 minutes. Add the peppers, asparagus and onion, season with salt and pepper and cook until warmed through. Mound the spaetzle and vegetables on 4 large plates and set the steaks on top with some spoonfuls of the sauce. Serve at once.