Preheat oven to 400°F. Wash and chop the peaches (no need to peel — the skin softens during roasting). Toss them with 2 tbsp white sugar and the lemon juice on a lined baking sheet. Spread in an even layer. Roast for 20-25 minutes until bubbly, juicy, and slightly caramelized around the edges.
Let the peaches cool completely on the baking sheet. If they're even slightly warm, they'll deflate your whipped cream. Pop them in the fridge if you're in a hurry.
Once cool, use a fork to mash the peaches. Leave some good-sized chunks — those little frozen nuggets are the best part.
In a large bowl, beat the heavy cream and vanilla extract until stiff peaks form. Don't overdo it, or you'll end up with butter!
Pour the sweetened condensed milk over the whipped cream. Gently fold with a rubber spatula until combined and smooth. Be gentle to keep as much air in as possible.
Add the mashed peaches (and any juice from the pan). Fold it in just 3-4 times — you want big, dramatic ribbons of peach running through the white base, not a fully pink mixture.
Scrape the mixture into your loaf pan. Smooth the top. Press a piece of plastic wrap directly onto the surface of the ice cream, then cover tightly with lid or foil. Freeze for at least 6 hours, preferably overnight.
Let it sit on the counter for 10-15 minutes before scooping. This makes all the difference in texture.