In a large saucepan over medium heat, bring milk to a simmer. Slowly whisk in semolina flour. Continue whisking until thickened, about 2 minutes. If too thick, continue to add milk until desired consistency.
Remove from heat and stir in butter until melted, then add mascarpone and parmesan cheese.
Season to taste with salt and pepper. Transfer polenta to a serving bowl. Top with extra parmesan, and tons of freshly cracked black pepper. Garnish with fresh thyme.