Substitute for Dijon mustard and if so, what is it

Well, the answer is "it depends." This is pretty much the same as asking, "I have sugar, and I want sugar syrup. How much water do I add?" It depends on the concentration you're looking for. If you're looking for a 24% solution, it's 24 grams of glucose in 76 grams of water. A 30% solution is 30 grams of glucose in 70 grams of water, etc, etc.

Unless you have some chemical reason to avoid dextrin, you can just substitute corn syrup. The only reason they use glucose in Europe is because they don't have our superabundance of corn.

I came across this article.

It explains where mustard get its heat from:

Mustard seeds come from the mustard plant, a member of the cabbage family. They contain two sulphur compounds, myrosin and sinigrin, as well as an enzyme, myrosinase. When the seeds are broken and water is added, the enzyme breaks down the sulphur compounds. The result is the sharp tasting oil that gives mustard its pungency, and helps explain why the name mustard comes from the Latin words mustum (much) and ardens (burning).

In addition, it recommends adding some flour to tone down its heat.

In the case of prepared mustards, the reaction is toned down by using additives such as flour.