Tip: When pasta is cooked, make sure you reserve the pasta water as directed in Step 1 before draining.
Add 2 ½ (5 litres for 4 servings) litres salted water to a large pot and bring to a boil. Add pasta and cook for 8-10 minutes, or until al dente. Reserve ½ cup (1 cup for 4 servings) pasta water. Drain well and set aside.
Wash spinach and lemon. Roughly chop spinach. Cut lemon in half. Drain and pat flat iron steak dry on paper towel. Season both sides with salt and cracked black pepper.
Cook Steak + Make Glaze
Heat a skillet over medium-high. Turn heat to medium. Add 1 tbsp. (2 tbsp. for 4 servings) olive oil to skillet. Add steak. Cook for 2 ½ - 3 minutes per side, or until an internal temperature of 135°F. To make glaze, add 1 tbsp. (2 tbsp. for 4 servings) butter and balsamic vinegar to skillet. Swirl glaze around as it thickens and coats steak. Remove steak from skillet. Let rest until internal temperature is 145°F. Reserve glaze for serving.
Add 2 tbsp. (4 tbsp. for 4 servings) butter to same large pot over medium heat. When melted, add garlic and stir until fragrant. Add pasta and reserved pasta water back to pot. Add spinach. Cook until bright green and tender. Season to taste with red pepper flakes, parmesan, and lemon juice.
Divide pasta onto plates. Cut steak diagonally against the grain into ¼ inch slices. Serve on top of pasta. Drizzle leftover glaze on steak. Enjoy!