Instructions:
- Heat the oil in a medium skillet over medium heat. Add the onion, fennel, garlic, and a few pinches of salt and pepper. Cook, stirring occasionally, until the onions and fennel are golden brown around the edges, 8 to 10 minutes. Stir in the balsamic and cook 2 more minutes until the fennel is very soft and the onion is lightly caramelized. (If necessary, reduce the heat and add a little more oil to prevent the onions from burning). Add the sage and stir.
- Bring a large pot of salted water to a boil. Prepare the pasta according to the instructions on the package, cooking until al dente. Do not drain.
- Remove the crushed garlic cloves from the onion mixture, then scoop the pasta into the skillet directly from the water. Add the tatsoi leaves and toss until everything is incorporated and the tatsoi is wilted. Add ¼ cup of the starchy pasta water, if necessary, to help loosen the pasta. Season to taste with more salt, pepper, balsamic, and a squeeze of lemon, if desired.
- Serve in bowls with toasted walnuts and Parmesan shavings, if using.
Make this vegan by using vegan pasta and omitting the parmesan cheese. Make it gluten free by using gluten-free pasta.