Heat the oven to 220 C / 425 F and line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment.
Halve the radishes and toss them in a mixing bowl with 1 tbsp olive oil, the salt, and pepper. Spread them cut-side down on the sheet pan.
Roast for 15 to 18 minutes, flipping once, until the edges are caramelized and a fork slides easily into the center.
While the radishes roast, trim the green beans and pat them dry.
Heat the remaining 1 tbsp olive oil in a 26 cm / 10-inch skillet over medium-high heat. Add the green beans and saute for 5 to 6 minutes, stirring occasionally, until bright green with blistered spots and tender-crisp.
Add the minced garlic to the skillet and cook for 30 seconds, stirring, until fragrant. Watch closely so it doesn't burn.
Add the roasted radishes to the skillet with the beans. Toss to combine and cook for 1 to 2 minutes to warm everything through.
Remove the skillet from the heat. Stir in the lemon zest, lemon juice, and red pepper flakes if using.
Taste and adjust salt and pepper. Scatter the chopped parsley over the top and serve warm.