In warm soapy water, wash and dry your glass jars and lids. You can use any size jars that you have handy as long as they have a tight fitting lid.
Thinly slice the cucumbers and onion using the slicing attachment from a food processor. You may cut the vegetables by hand, but be sure to get them very thin with a uniform consistency. If they are different thicknesses, they will absorb unequal amounts of the seasonings.
Layer the cucumber, onion, garlic and green pepper (optional sliced jalapeno) in your jar; making three layers, ending with the cucumber as the top layer. Firmly press the layers into the jar to pack in as much as possible. Leave 1“space (headroom) at the top of the jar.
In a medium sauce pan, whisk together the vinegar, sugar, salts, peppers and mustard seed (optional). Cook over medium high heat, stirring occasionally until the sugar is dissolved into the brine. Remove from heat.
Slowly and carefully pour the warm liquid into the jar, allowing it to be absorbed in the spaces and by the veggies until the jar is full. Tighten the lid. Refrigerate overnight to allow the flavors to mix.