For the jam: Place the diced tomatoes in a medium saucepan over medium-high heat and bring to a boil. Reduce the heat to a simmer and add the strawberries, sugar, cinnamon, salt and lemon juice. Stir to combine. Bring to a simmer and cook over medium-low heat, stirring occasionally, until the mixture has thickened to the consistency of a loose jam, 40 to 45 minutes. Set aside to cool completely.
To assemble the crostini: Preheat the broiler to high.
Slice the baguette into 1/4-inch-thick rounds. Brush each slice on both sides with the olive oil. Place the bread on a baking sheet, sprinkle with the salt and broil until golden brown and crispy, 5 to 6 minutes. Top each crostini with a teaspoon of jam and some of the feta. Garnish with one or two mint leaves if using.
Refrigerate any leftover jam in an airtight container for up to a week.