Adjust oven rack to middle position and preheat oven to 350° F. Place frozen meatballs on one side of the baking sheet.

Bring milk and 3 cups water to a boil over medium-high heat in a medium saucepan. Reduce heat to medium. Whisking constantly, gradually add polenta; bring to a simmer. Reduce heat to low, cover pan, and cook, whisking every 10–15 minutes, until thickened and no longer gritty, 30–35 minutes. Remove from heat and add Parmesan, butter, and salt. Cook, whisking, until butter and cheese are melted, and polenta is the consistency of porridge, about 1 minute.

Combine all Chimichurri ingredients in the bowl of a food processor. Except the liquids. Pulse until all ingredients are finely chopped.

Transfer Chimichurri to a medium bowl and stir in olive oil, vinegar, and lemon.

Transfer polenta to serving bowl or cast-iron skillet top with grated parmesan cheese and cooked meatballs. 

Garnish with the chimichurri and serve with grilled bread.