Melt 2 tablespoons butter in 12-inch nonstick skillet
over medium-high heat. Add onion, garlic, thyme,
and ½ teaspoon salt and cook, stirring often, until
onion begins to brown, about 5 minutes. Stir in broth,
lentils, and lemon juice and bring to boil. Reduce
heat to low, cover, and cook until lentils are tender,
to 30 minutes.
Uncover and cook, stirring often, until most liquid
has evaporated, about 2 minutes. Season with salt
and pepper to taste, transfer to bowl, and cover to
keep warm.
Pat salmon dry with paper towels and season with
salt and pepper. Wipe out skillet with paper towels,
add oil, and heat over medium-high heat until just
smoking. Carefully lay salmon in skillet, skin side
up, and cook until well browned on first side, 4 to
minutes.
Flip salmon over, reduce heat to medium, and cook
until center is still translucent when checked with
tip of paring knife and registers 125 degrees (for
medium-rare), 3 to 6 minutes. Transfer salmon to
clean plate, tent loosely with aluminum foil.
Add lentil mixture to now-empty skillet and cook
over medium-high heat until hot, about 4 minutes.
Stir in chard and remaining 1 tablespoon butter and
cook until chard is wilted, 2 to 3 minutes. Serve
with salmon and lemon wedges.