For the Clarified Butter
Heat butter in a small pot over medium-low heat until melted. Remove from heat and let sit 5 minutes.
Slowly and carefully pour or ladle out butter into bowl, until you reach the milky solids (which can be discarded). Use butter or chill for later use.
For the Croutons
Preheat oven to 180°C and line a baking tray with parchment paper. Toss bread with butter, garlic, parsley and season.
Spread bread onto prepared baking tray and toast for 12 to 18 minutes, stirring once or twice, until golden brown. Cool and store in a resealable container until ready to use.
For the Trout
Keep oven temperature at 180°C and line a baking tray with parchment paper. Heat a large sauté pan over medium heat and add 1 tablespoon melted clarified butter.
Add leeks and thyme and cook, stirring often, until leeks are tender, about 5 minutes. Season lightly and remove from pan. Increase heat to medium high and add 2 tablespoons butter. Season trout lightly and place 2 fillets, skin-side up in pan.
Cook for 4 minutes, until fish has browned, then gently remove onto prepared baking tray, skin-side down.
Add 2 more tablespoons of butter and repeat with 2 more fillets, then repeat process with last 2 fillets. Place baking tray in oven and roast for about 10 minutes, until fish yields when touched with a fork.
To finish sauce, cook remaining butter in pan over medium heat until browned (it may be a little browned by now). Add leeks to warm. Add juice of 1 lemon, parsley and salt & pepper. Stir in croutons immediately before serving.
To serve, place trout on a plate or platter, and top with sauce. Serve immediately.