Heat the olive oil in a medium saucepan. Add the onion and saute over medium heat until tender, about 5 minutes. As the onion cooks, finely chop the parsley, then set it aside.
Add the rice to the onion and stir until all the grains are coated evenly with the oil, about 1-2 minutes. Add the water, salt and black pepper. Stir to combine and bring to a boil. Reduce the heat to medium and simmer, covered, until the rice is tender, about 18-20 minutes.
Stir in the parsley, cover and let the pilaf stand 5 minutes before serving.
As the rice cooks, place the pecans in a food processor and pulse until finely ground but not sticky. Combine with the breadcrumbs. Place the trout opened, skin-side down, on a sheet pan lined with aluminum foil. Season the flesh with salt and pepper, sprinkle with the pecan mixture and the butter pieces, then place in the oven. Cook until the edges of the trout skin on the bottom are crisp and the thick part of the flesh is opaque, about 5-8 minutes. Increase the heat to broil and place the pan near the broiler element for a few minutes until the topping is golden brown.
Remove from the oven, cut each trout into 2 filets and serve each person 1-2 fillets on a bed of pilaf with the lemon wedges.