In a small to medium pot, bring rhubarb and sugar to a simmer over medium/low heat. Simmer until rhubarb is falling apart and is the texture of apple sauce, about 15 minutes. Remove from heat and stir in the lemon juice. Puree with an immersion blender or by pouring into a regular blender. If using a regular blender, make sure there is room for the steam to escape! (Pureeing is optional; you can definitely leave the sauce chunky if you prefer.) Set aside.
Rinse salmon fillets and pat dry with paper towels. Sprinkle the tops with salt and pepper.
Heat oil in a large pan over medium heat. Make sure to let it get nice and hot so that the fish won't stick - the oil should be shimmering. Do NOT let the oil get too hot, though. If the oil is sizzling or smoking, remove from heat, reduce heat, and wait several minutes before adding the fish. (Hot oil splatters are no joke, guys! I recommend an apron and long sleeves.) Once the pan is the right temperature, use a pair of tongs place the salmon fillets top side down in the pan. Let cook without moving for 4-5 minutes, then sprinkle with salt and pepper, flip, and cook another 4-5 minutes. Salmon should be crispy on both sides and cooked all the way through.
Remove salmon from pan and let drain on paper towels for a minute or so. Serve and spoon rhubarb sauce over the top of the fillets.