Heat the olive oil and butter in a large skillet or pot over medium heat.
Add the chopped onion and minced garlic to the pot and sauté for 2-3 minutes until the onion becomes translucent and fragrant.
Place the salmon fillets in the pot and cook for 3-4 minutes on one side without moving them.
Carefully flip the salmon fillets and cook for another 3 minutes on the other side.
Add the fresh spinach to the pot, stirring gently to combine with the salmon and onions. Cook until the spinach wilts, about 2 minutes.
Pour the heavy cream over the salmon and spinach mixture. Stir gently to combine all ingredients.
Season with salt, black pepper, and lemon juice. Reduce the heat to low and simmer for 5-6 minutes until the salmon is cooked through and the sauce slightly thickens.
Remove from heat and serve immediately.