For the strawberries, wash, pat dry, hull and slice. In a medium non-reactive bowl, toss strawberries and sugar together. Let it macerate for 15 minutes.
In the meantime, prepare the rhubarb. Trim off ends and make sure to remove all leaves. Cut into ½-inch pieces.
In a medium non-reactive saucepan, add rhubarb, lemon zest & lemon juice. Bring to a boil, then reduce to a simmer. Cover with a lid partially, and cook until the rhubarb has softened and broken apart completely, about 10 minutes, stirring often.
Now, add the strawberries with sugar (including any liquid). Add vanilla bean paste or extract. Bring to a boil, and reduce heat to medium. Cook until mixture is reduced and thickened, about 20 minutes, stirring often.
Carefully transfer the jam to clean heat-proof containers or glass jars (with lid). Cool to room temperature. Cover with lid and store in the fridge.
Notes
Rhubarb is in season during spring time from April to June in the US. Pick ones that are firm with no bruises and not too thick. Thicker ones are more fibrous. Colors don’t matter too much.
Non-reactive materials mean ceramics, enamel, glass, plastic, or stainless steel. Reactive materials, like aluminum, will react to the acid in food and have a metallic taste.