The diaspora factor
Düsseldorf, London, and Paris have the largest Japanese expat communities in Europe — 8,000+ Japanese residents in Düsseldorf alone (centered around Immermannstraße). Restaurants serving expats by definition serve more rigorously authentic food than tourist-targeted sushi.
Düsseldorf
Düsseldorf "Little Tokyo" along Immermannstraße — Naniwa (ramen), Takumi, Maruyasu (sushi). Family-run lunch counters serving udon and tonkotsu at €10–14. Dinner izakayas €30–50.
Paris
Paris Sushi B (3 Michelin stars), Jin (1 star), Ze Kitchen Galerie. Rue Sainte-Anne is the historic Japanese quarter — over 30 ramen and udon shops. Many tired tourist sushi bars exist; serious ones cluster near Opéra/Pyramides.
London
London Endō at the Rotunda (Michelin), Roketsu, Sushi Tetsu, Koya (handmade udon), Ippudo. Dense Japanese-restaurant map; quality varies enormously. Soho and Mayfair dense.
Berlin
Berlin ramen-and-izakaya scene matured rapidly — Cocolo Ramen, Cocoro, Saigaiya. Lower prices than London or Paris (€11–15 ramen). Mitte base.
Frankfurt
Frankfurt business-Japan diaspora — sushi at Iwase, ramen at Tako (small). Smaller than Düsseldorf but day-trip distance.
Strategy
Lunch-set strategy at high-end sushi (€30–80) far cheaper than dinner counter (€150–400). Reserve omakase counters 2–4 weeks ahead. Michelin guide for the splashy companion.