Pat salmon chunks dry with paper towels. Season with half the salt and all the pepper. Roughly chop the spinach if the leaves are large - you want bite-sized pieces that wilt evenly.

Heat olive oil in a large skillet over medium-high heat until shimmering. Add salmon chunks in a single layer and cook 2-3 minutes without moving, until golden on the bottom. Flip once and cook 1 minute more - the centers should still be slightly translucent. Transfer to a plate; the salmon will finish cooking in the oven.

Return the same skillet to medium heat (no need to wipe it out - those salmon bits add flavor). Add spinach in batches, stirring until just wilted, about 3 minutes total. It will look like too much at first but cooks down dramatically.

Reduce heat to medium-low. Push spinach to the sides and add garlic to the center; cook 30 seconds until fragrant. Add cream cheese and heavy cream, stirring until the cream cheese melts into a smooth sauce, about 4 minutes. Stir in lemon juice and remaining 1/4 teaspoon salt. The sauce should coat the back of a spoon.

Preheat oven to 375 degrees F. Spread the spinach-cream mixture evenly in a 9x13 inch baking dish. Nestle the seared salmon chunks throughout, pressing them slightly into the sauce. Top with mozzarella, then sprinkle Parmesan over everything.

Bake uncovered for 20-25 minutes, until the cheese is melted and spotty golden brown in places, and the sauce bubbles at the edges. Let rest 5 minutes before serving - the sauce thickens as it sits.