Pat salmon dry and season with salt, pepper, and oregano.
Heat 1 tbsp olive oil in a skillet. Sauté zucchini and bell pepper for 3-4 minutes, then add spinach and cook until wilted. Set aside.
Add the remaining olive oil to the skillet and cook salmon fillets for 4-5 minutes per side.
Add garlic to the skillet, sauté for 30 seconds, then add lemon juice, zest, and optional butter. Stir to combine.
Return the vegetables to the skillet, toss in the sauce, and place the salmon back in.
Garnish with extra lemon and serve.