In a medium heavy-bottom pot over medium heat, melt 4 tablespoon butter with 1 tablespoon olive oil. Once the butter is fully melted and looks foamy, stir in the garlic and cook until fragrant, 30 seconds. Do not let the garlic brown.
Add the rice and stir to coat every grain in the butter. Keep stirring until the rice smells lightly toasty and the pot smells like buttered popcorn, 2 minutes. This quick toasting step is the difference between "simple rice recipes" that taste flat and rice that tastes like it came from a restaurant.
Pour in the chicken broth, then stir in ½ teaspoon kosher salt and ¼ teaspoon black pepper. Turn the heat to high and bring to a boil.
As soon as it boils, reduce the heat to low, cover with a tight-fitting lid, and simmer for 15 minutes. Try not to lift the lid. Steam is doing the work here.
After 15 minutes, turn off the heat and let the rice rest, covered, for 10 minutes. This rest finishes the cook gently and keeps the bottom from getting gummy.
Uncover and fluff with a fork, gently lifting and separating the grains. Add the remaining 2 tablespoon butter and fold until glossy. Then sprinkle in the parmesan a little at a time, folding after each addition. Off-heat folding is the best way to keep the cheese creamy and evenly distributed.
Finish with lemon zest (if using) and parsley. Taste and adjust with a pinch more salt or pepper. Serve hot, while the butter and parmesan are at peak melt.