Cook pasta according to package directions; drain and reserve 1/4 cup of pasta water for the sauce.
In a sauté pan, heat 2 tablespoons olive oil on medium-high heat.
Add Brussels sprouts; cook untouched for 3-4 minutes until nicely charred.
Stir in garlic and cook for another 2-3 minutes, stirring occasionally.
Transfer fettuccine and reserved pasta water to the sauté pan.
Bring to a boil and reduce water by half.
Remove from heat; stir in remaining olive oil, mint, lemon juice, lemon zest, salt, pepper and chili flakes, if using.
Toss gently to combine and serve immediately.