Split Romanesco into florets and wash. Peel and halve the onion and cut into thin slices. Dab the meat dry. Heat the oil in a pan. Fry the meat for about 2 minutes while turning it.
Season with salt and pepper and place on a baking tray. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes in medium heat. Cook Romanesco in boiling salted water for 6-8 minutes.
Add the onion to the frying pan and fry over medium heat for about 2 minutes, turning. Deglaze with the vegetable stock, bring to the boil and simmer for about 10 minutes. Prepare spaetzle in boiling salted water according to package instructions, drain and drain.
Stir the starch with a little water until smooth. Thicken the sauce with it. Season with salt and pepper. Drain the Romanesco. Arrange fillet of beef, Romanesco, spaetzle and sauce on plates. Sprinkle beef fillet with coloured pepper.