Start by blanching the ripe summer tomatoes to make peeling easier. Cut an 'X' on the bottom of each tomato, then submerge them in boiling water for 20-30 seconds. Remove them immediately and place in ice water to cool.

Once peeled, dice the tomatoes and onions into small pieces. In a large bowl, sprinkle the diced mixture generously with salt. Cover the bowl with plastic wrap and let it sit overnight at room temperature.

The next day, strain the tomato and onion mixture using a colander over a large bowl. Allow it to drain for about 30 minutes or until excess liquid is removed.

In a medium saucepan, combine the strained tomatoes and onions with vinegar, sugar, and mustard powder. Stir the mixture well until everything is incorporated.

Place the saucepan over medium-high heat and bring to a boil. Reduce the heat to low and let it simmer uncovered for about 1 hour, stirring occasionally.

Remove from heat once the relish has thickened. Pour the hot Tomato Relish into sterilized jars, seal tightly, and let cool to room temperature.