Toast walnuts in a dry skillet over medium-low heat for 1-2 minutes until fragrant. Remove to a bowl.
Add olive oil and minced garlic to the skillet. Sauté for 1 minute until fragrant. Add frozen broccoli and sauté for 2 minutes. Remove broccoli to the bowl with walnuts.
Add bowtie pasta, vegetable broth, sun-dried tomatoes, lemon juice, and red pepper flakes to the skillet. Stir. Cover and bring to a boil over high heat. Stir, reduce heat to low, cover, and simmer for 7-10 minutes until pasta is tender and broth is absorbed. Stir occasionally.
While pasta simmers, zest the lemon. Once pasta is cooked, return broccoli and walnuts to the skillet. Stir to combine. Top with crumbled feta and lemon zest. Serve immediately.