With the rack in the middle position, preheat the oven to 450°F (225°C). Line a baking sheet with parchment paper.
On the sheet, toss the bell peppers, celery and onion with the oil. Season with salt and pepper. Bake for about 35 minutes, stirring halfway through cooking, or until the vegetables are tender. Add the spinach, combine with the vegetables and continue cooking for 5 minutes or until the spinach has wilted.
Meanwhile, in a non-stick skillet over high heat, brown the fish in 2 tbsp of the butter for about 3 minutes on each side or until the desired doneness according to the thickness of the steaks. Season with salt and pepper. Set aside on a plate. Wipe the skillet to remove the burnt butter.
In the same skillet over medium heat, add the remaining butter (6 tbsp/90 g) and cook for 3 to 5 minutes or until it turns golden brown. Quickly remove the brown butter from the heat and strain through a sieve over a bowl (see note). Add the capers and anchovy paste.
Divide the vegetables and fish among the serving plates. Drizzle with the caper and anchovy butter.