Bring 4 cups of water to a near boil (about 200°F / 93°C). Remove from heat and add 4 tea bags (or 4 tablespoons loose black tea in an infuser). Steep for 4–5 minutes for a medium-strength brew (steep up to 6 minutes for stronger tea). Remove the tea bags or strain the leaves and discard.

Make Raspberry Syrup

While the tea steeps or immediately after, combine 2 cups raspberries and 1 cup granulated sugar in a small saucepan with 1/4 cup of the brewed hot tea (or 1/4 cup hot water). Cook over medium heat, stirring occasionally, until the raspberries break down and the sugar dissolves, about 6–8 minutes. Use a spoon or potato masher to press raspberries against the side of the pan to release juice. If you prefer a smoother syrup, simmer an extra 1–2 minutes.

Strain Syrup

Pour the raspberry mixture through a fine mesh sieve set over a bowl, pressing on solids to extract as much liquid as possible. Discard seeds and pulp, reserving the clear raspberry syrup. You should have roughly 1 to 1 1/2 cups of syrup depending on fruit ripeness. If syrup is too thick, stir in up to 1/2 cup cold water to reach desired pouring consistency.

Sweeten and Combine

While the brewed tea is still warm, stir in the strained raspberry syrup until evenly combined. Taste and add lemon juice (2 tablespoons). If you want it sweeter, add more syrup or up to 1/4 cup additional granulated sugar dissolved in a couple tablespoons hot tea. If the combined tea is too strong, add up to 1/2 cup cold water to adjust strength.

Chill

Transfer the combined tea to a pitcher and let cool to room temperature, about 20–30 minutes, then refrigerate until well chilled, at least 1 hour. (For faster cooling, place the pitcher in an ice bath or stir in a large handful of ice and then pour off melted ice before refrigerating.)

Serve

Fill glasses with ice cubes. Pour chilled raspberry iced tea over ice, garnish each glass with a lemon slice and a mint sprig if using. Stir before drinking. Keeps in the refrigerator in a covered pitcher for up to 3 days; stir or shake gently before serving as separation may occur.